THRESHOLD PHOTO IONISATION AND DENSITY FUNCTIONAL THEORY STUDIES by mikesanye

VIEWS: 1 PAGES: 59

									   THRESHOLD PHOTO-IONISATION AND
 DENSITY FUNCTIONAL THEORY STUDIES
       OF METAL-CARBIDE CLUSTERS


                      Viktoras Dryza


A thesis submitted in total fulfillment of the requirements for
            the degree of Doctor of Philosophy




                      November, 2008


                  Department of Chemistry
                 The University of Adelaide
This work contains no material which has been accepted for the award of any other
degree or diploma in any university or other tertiary institution and, to the best of my
knowledge and belief, contains no material previously published or written by another
person, except where due reference has been made in the text.


I give consent to this copy of my thesis, when deposited in the University Library, being
made available in all forms of media, now or hereafter known.




                                                                         Viktoras Dryza
                                                                       November, 2008.




                                           II
Dedicated to my Dad.




        III
        “Serenity now, serenity now!”
- Frank Costanza (Seinfeld, Season 9/Episode 3)




                      IV
                                       Abstract

Neutral gas-phase metal-carbide clusters are generated by laser ablation and are detected
in the constructed time-of-flight mass-spectrometer by laser ionisation. Photo-ionisation
efficiency (PIE) experiments are performed on the metal-carbide clusters to determine
their ionisation potentials (IPs). Complimentary density functional theory (DFT)
calculations are performed on the energetically favorable structural isomers of the metal-
carbide clusters. Comparison between the calculated IPs of the isomers and the
experimental IP allows the carrier of the observed ionisation onset for a metal-carbide
cluster to be assigned.


The niobium-carbide clusters Nb3Cy (y = 0–4), Nb4Cy (y = 0–6) and Nb5Cy (y = 0–6) are
examined by PIE experiments and DFT calculations. The IPs of the niobium-carbide
clusters are found to be either left reasonably unchanged from the IPs of the bare metal
clusters or moderately reduced. The clusters Nb3C2, Nb4C4, Nb5C2 and Nb5C3 display the
largest IP reductions for their corresponding cluster series.


The structures assigned to the IPs of the Nb3Cy (y = 1–3) clusters are based on the carbon
atoms attaching to the niobium faces and/or niobium-niobium edges of the triangular Nb3
cluster. However, for Nb3C4 the ionisation onset is assigned to a low-lying isomer, which
contains a molecular C2 unit, rather than the lowest energy isomer, a niobium atom
deficient 2×2×2 face-centred cubic (fcc) nanocrystal structure.


The structures assigned to the IPs of the Nb4Cy (y = 1–4) clusters are based on the carbon
atoms attaching in turn to the niobium faces of the tetrahedral Nb4 cluster, developing a
2×2×2 fcc nanocrystal structure for Nb4C4. For Nb4C3 two ionisation onsets are observed;
one weak onset at low energy and another more intense onset at high energy. It is
proposed that the two onsets are due to ionisation from both a metastable 3A1 state and the
ground 1A1 state of the lowest energy isomer. The ionisation onsets of Nb4C5 and Nb4C6
are also proposed to originate from metastable triplet states of the lowest energy isomers,
with the transitions from the ground singlet states calculated to be greater than the highest


                                              V
achievable photon energy in the laboratory. The structures of Nb4C5 and Nb4C6 have one
and two carbon atoms in a 2×2×2 fcc nanocrystal substituted with molecular C2 units,
respectively.


The structures assigned to the IPs of the Nb5Cy (y = 1–6) clusters are based on the
underlying Nb5 cluster being in either a “prolate” or “oblate” trigonal bipyramid
geometry; the former has six niobium faces available for carbon addition, while the latter
has two niobium butterfly motifs and two niobium faces available for carbon addition.
Both the structures of Nb5C5 and Nb5C6 have the underlying Nb5 cluster in the oblate
trigonal bipyramid geometry and contain one and two molecular C2 units, respectively.


The tantalum-carbide clusters Ta3Cy (y = 0–3), Ta4Cy (y = 0–4) and Ta5Cy (y = 0–6) are
examined by PIE experiments and DFT calculations. The IPs of the tantalum-carbide
clusters in each series show trends that are very similar to the corresponding iso-valent
niobium-carbide cluster series, although the IP reductions upon carbon addition are
smaller for the former. For the vast majority of tantalum-carbide clusters, the same
structural isomer is assigned to the ionisation onset as that assigned for the corresponding
niobium-carbide cluster.


Bimetallic tantalum-zirconium-carbide clusters are generated using a constructed double
ablation cluster source. The Ta3ZrCy (y = 0–4) clusters are examined by PIE experiments
and DFT calculations. The IP trend for the Ta3ZrCy cluster series is reasonably similar to
that of the Ta4Cy cluster series, although the IP reductions upon carbon addition are
greater for the former. The structures assigned to the IPs of the Ta3ZrCy (y = 1–4) clusters
are based on the carbon atoms attaching in turn to the metal faces of the tetrahedral Ta3Zr
cluster.


In summary, the work presented in this thesis demonstrates that the structures of metal-
carbide clusters can be inferred by the determination of their IPs through PIE experiments
in combination with DFT calculations on candidate structural isomers.




                                            VI
                                  Acknowledgments

First and foremost I would like to thank my supervisor Dr. Greg Metha for giving me this
opportunity. Professionally his helpful advice, extensive knowledge and forward thinking
attitude to research have been invaluable. Personally he has been a great friend and mentor. I
will always remember our stay in Munich; getting lost in a dark snowing forest and throwing
snowballs at each other on the way home after a late night out. Also, the many laughing fits
at our mutual appreciation of Borat.


Thanks must also go to my co-supervisor Prof. Mark Buntine for his smart-arse comments
and the occasional helpful one.


I am also totally indebted to our post-doc Dr. Jason Gascooke, whose ability to solve
problems which seem unsolvable to me is truly amazing. He has also been a great drinking
buddy and is the co-inventor of beer-table tennis.


Thanks to Matt Addicoat for his help with the computational calculations.


The support in the mechanical workshop from Peter Apoefis has been fantastic in converting
my caveman-like drawings into reality.


I have also had many a fun time in the lab with my partner in crime Olivia Maselli; whether it
be dancing around like a fool, discussing our small overlap in musical tastes or laughing at
her stories. Also thanks to all the other Honours and PhD students I have shared the lab with.


Huge thanks must go to my extremely supportive family (Mum, Dad and Kristina) for the
Sunday lunches and for still believing I have some intelligence, despite observing my
constant stupidity over the years.


Last, but definitely not least, I must thank my beautiful girlfriend Milena. She has always
believed in me and has the magical ability to make me happy whenever I’m with her (even
when my experiments are not working).



                                             VII
                                   Publications

The following publications were produced from the work presented in this thesis:


Ionization Potentials of Tantalum-Carbide Clusters: An Experimental and Density
Functional Theory Study
Dryza, V.; Addicoat, M.A.; Gascooke, J.R.; Buntine, M.A.; Metha, G.F. J. Phys. Chem. A
2005, 109, 11180.


Threshold Photo-ionization and Density Functional Theory Studies of the Niobium-
Carbide Clusters Nb3Cn (n = 1–4) and Nb4Cn (n = 1–6)
Dryza, V.; Addicoat, M.A.; Gascooke, J.R.; Buntine, M.A.; Metha, G.F. J. Phys. Chem. A
2008, 112, 5582.


Onset of Carbon-Carbon Bonding in the Nb5Cy (y = 0–6) Clusters: A Threshold Photo-
ionisation and Density Functional Theory Study
Dryza, V.; Gascooke, J.R.; Buntine, M.A.; Metha, G.F. PCCP 2008, (in press).




The following publications are currently in preparation from the work presented in this
thesis:


Onset of Carbon-Carbon Bonding in the Ta5Cy (y = 0–6) Clusters: A Threshold Photo-
ionization and Density Functional Theory Study
Dryza, V.; Gascooke, J.R.; Buntine, M.A.; Metha, G.F. (in preparation).


Threshold Photo-ionization and Density Functional Theory Studies of Bimetallic-Carbide
Clusters: Ta3ZrCy (y = 0–4)
Dryza, V.; Gascooke, J.R.; Buntine, M.A.; Metha, G.F. (in preparation).




                                          VIII
                                  Abbreviations

AO         Atomic Orbital
C          Carbon
DFT        Density Functional Theory
EA         Electron Affinity
ECP        Effective Core Potential
eV         Electron Volt
FC         Frank-Condon
FCC        Face-centred Cubic
FEL        Free Electron Laser
FWHM       Full Width at Half Maximum
GTO        Gaussian Type Orbital
HOMO       Highest Occupied Molecular Orbital
IP         Ionisation Potential
IR         Infrared
LUMO       Lowest Unoccupied Molecular Orbital
MO         Molecular Orbital
MPD        Multi-photon Dissociation
MPI        Multi-photon Ionisation
MRCI       Multi-Reference Configuration Interaction
Nb         Niobium
NBO        Natural Bond Order
PES        Potential Energy Surface
PFI-ZEKE   Pulsed Field Ionisation Zero Electron Kinetic Energy
PIE        Photo-ionisation Efficiency
REMPI      Resonance Enhanced Multi-photon Ionisation
SPI        Single-photon Ionisation
Ta         Tantalum
TOF-MS     Time-of-flight Mass-spectrometer
ZPE        Zero-point Energies
Zr         Zirconium



                                         IX
                            Table of Contents

      Chapter 1: Introduction to Metal-Carbide Clusters                  1
1.1 Generation and Distribution of Metal-Carbide Clusters                1
1.2 The Structures of the Ti8C12 “Metcar” and Ti14C13 “Nanocrystal”      3
1.3 Potential Applications of Metal-Carbide Clusters                     5
1.4 Scope of this Thesis                                                 7
1.5 Why Determine the Ionisation Potentials of Metal-Carbide Clusters?   8
1.6 References                                                           10


      Chapter 2: Experimental Approach                                   12
2.1 Laser Ablation                                                       12
      2.1.1 Laser Ablation Theory                                        12
      2.1.2 Experimental Design and Operation                            14
2.2 Photo-ionisation Efficiency (PIE)                                    17
      2.2.1 Photo-ionisation Theory                                      17
      2.2.2 PIE Theory                                                   21
      2.2.3 PIE of Transition Metal Containing Clusters                  23
      2.2.4 Experimental Procedure for PIE Experiments                   26
2.3 References                                                           31


      Chapter 3: Computational Approach                                  33
3.1 Introduction to Density Functional Theory (DFT)                      33
3.2 Basis Sets                                                           35
3.3 Computational Method and Procedure for Metal-Carbide Clusters        37
3.4 References                                                           40


      Chapter 4: The Time-of-Flight Mass-Spectrometer                    42
4.1 The Constructed TOF-MS Apparatus                                     42
4.2 References                                                           47



                                        X
      Chapter 5: Niobium-Carbide Clusters                           48
5.1 Introduction to Niobium-Carbide and Tantalum-Carbide Clusters   48
5.2 Photo-ionisation Efficiency Experiments                         52
5.3 DFT Calculated Isomers                                          57
      5.3.1 The Nb3Cy (y = 0–4) Cluster Series                      57
      5.3.2 The Nb4Cy (y = 0–6) Cluster Series                      60
      5.3.3 The Nb5Cy (y = 0–6) Cluster Series                      65
5.4 Comparison between Calculated and Experimental IPs              74
      5.4.1 The Nb3Cy (y = 0–4) Cluster Series                      74
      5.4.2 The Nb4Cy (y = 0–6) Cluster Series                      77
      5.4.3 The Nb5Cy (y = 0–6) Cluster Series                      81
5.5 Discussion on Niobium-Carbide Clusters                          85
      5.5.1 Comparison to Previous Spectroscopic Data               85
      5.5.2 Onset of Carbon-Carbon Bonding                          89
      5.5.3 Low-Lying Isomers and Metastable Electronic States      90
      5.5.4 IP trends, Molecular Orbitals and HOMO-LUMO gaps        94
      5.5.5 Thermodynamic Cycle                                     103
5.6 Summary and Future Directions                                   105
5.7 References                                                      109


      Chapter 6: Tantalum-Carbide Clusters                          112
6.1 Photo-ionisation Efficiency Experiments                         112
6.2 Comparison between Calculated and Experimental IPs              116
      6.2.1 The Ta3Cy (y = 0–3) Cluster Series                      117
      6.2.2 The Ta4Cy (y = 0–4) Cluster Series                      119
      6.2.3 The Ta5Cy (y = 0–6) Cluster Series                      122
6.3 Discussion on Tantalum-Carbide Clusters                         127
      6.3.1 Comparison to the Niobium-Carbide Clusters              127
6.4 Summary                                                         129
6.5 References                                                      130




                                       XI
      Chapter 7: Tantalum-Zirconium-Carbide Clusters                     131
7.1 Introduction to Bimetallic-Carbide Clusters                          131
7.2 Double Ablation Cluster Source Design                                133
7.3 Generation of Tantalum-Zirconium-Carbide Clusters                    135
7.4 Photo-ionisation Efficiency Experiments                              141
7.5 DFT Calculated Isomers                                               143
      7.5.1 The Ta3ZrCy (y = 0–4) Cluster Series                         143
7.6 Comparison between Calculated and Experimental IPs                   146
7.7 Discussion on Tantalum-Zirconium-Carbide Clusters                    149
      7.7.1 Relative Energetics of the Bimetallic-Carbide Cluster Isomers 149
      7.7.2 Electronic Structure of the Bimetallic-Carbide Clusters      149
      7.7.3 Comparison of IP Trends for the Ta3ZrCy and Ta4Cy Clusters 153
7.8 Summary                                                              156
7.9 References                                                           158


      Appendix I                                                         159
      Appendix II                                                        161
      Appendix III                                                       166
      Appendix IV                                                        189
      Appendix V                                                         193
      Appendix VI                                                        203
      Appendix VII                                                       205




                                        XII
Chapter 6: Tantalum-Carbide Clusters



Chapter 6: Tantalum-Carbide Clusters


6.1 Photo-ionisation Efficiency Experiments

A mass spectrum of tantalum-carbide clusters ionised at 210 nm, under the SPI
conditions used to conduct a PIE scan, is shown in Figure 6-1. Compared to the
corresponding mass spectrum of niobium-carbide clusters (see Section 5.2), the mass
distribution of tantalum-carbide clusters shows a lower amount of clustering for the
Ta atoms.



                                                                                                Ta C
                                                                                                  7    y
                                                                                   Ta C
                                                                                     6   y
     Intensity (Arb. Units)




                                  TaC
                                    y
                                                                      Ta C
                                                                           5   y




                                                          Ta C
                                                            4    y




                                               Ta C
                                                 3    y



                              0    200   400    600         800            1000          1200         1400   1600   1800
                                                           Mass (M/Z)

Figure 6-1: Mass spectrum of ionised neutral TaxCy clusters obtained by single-
photon ionisation (210 nm).


Parts a–e of Figure 6-2 show a portion of the mass spectra of tantalum-carbide
clusters following ionisation at five different wavelengths; 262, 245, 230, 220 and 210
nm, collected under otherwise identical conditions. In the spectrum recorded at 210
nm (Figure 6-2e), clusters containing Ta3 appear with 0–3 C atoms attached, clusters
containing Ta4 appear with 0–4 C atoms attached and clusters containing Ta5 appear
with 0–6 C atoms attached.


The spectrum recorded at 262 nm (Figure 6-2a) shows only baseline levels, as none of
the species are ionised with one photon. Following ionisation at 245 nm (Figure 6-2b)
both Ta5C2 and Ta5C3 increase in intensity. At 230 nm (Figure 6-2c) both Ta3C2 and


                                                                     112
Chapter 6: Tantalum-Carbide Clusters


Ta4C4 have dramatically increased in intensity, with the clusters Ta5C4, Ta5C5 and
Ta5C6 also appearing. By the wavelength 230 nm the clusters Ta3, Ta3C3, Ta4C2, Ta5
and Ta5C have appeared (Figure 6-2d). Finally back to 210 nm (Figure 6-2e), the
remaining clusters Ta3C, Ta4, Ta4C and Ta4C3 have increased from baseline intensity.


PIE spectra are recorded by monitoring the signal of each cluster as a function of
photon energy. PIE spectra for the Ta3Cy (y = 0–3), Ta4Cy (y = 0–4) and Ta5Cy (y = 0–
6) clusters are shown in parts a–d of Figure 6-3, a–e of Figure 6-4 and a–g of Figure
6-5, respectively. Nearly all clusters show a dramatic rise from their baseline levels.
Only Ta4C4 shows a gradual onset of ionisation displaying slight structure, suggesting
a notable geometry change between the neutral and cation. For all spectra two lines
are fitted; one to the baseline and one to the linear rise of signal, and their intersection
defined as the IP (with an estimated error of ±0.05 eV). This procedure was
previously described in Section 2.2.4. The determined IPs for all the tantalum-carbide
clusters considered in this study are displayed in the PIE spectra and are also given
later in the second column of Table 6-1, Table 6-2 and Table 6-3. No tantalum-
carbide clusters have previously had their IPs examined.


As a check, the ionisation energies extracted for the bare tantalum clusters Ta3, Ta4
and Ta5 are found to be in good agreement with those previously determined.1 Unlike
the iso-valent bare niobium clusters, where the IPs decrease with size, the Ta4 cluster
has the highest IP, followed by Ta3 and then Ta5. In the Ta3Cy series the addition of
one and three C atoms results in very little change in the IP, relative to that of Ta3; an
increase and decrease of 0.08 and 0.08 eV, respectively. The addition of two C atoms
results in an intermediate IP reduction of 0.35 eV. For the Ta4Cy series, addition of
one and three C atoms to Ta4 results in very slight decreases in the IP, by 0.01 and
0.05 eV respectively. However, addition of two C atoms results in an intermediate IP
reduction (−0.25 eV), while addition of four C atoms results in a large IP reduction
(−0.68 eV). In the Ta5Cy series addition of one C atom to Ta5 essentially leaves the IP
unchanged (an increase of 0.03 eV), yet addition of two and three C atoms results in
large reductions in the IP of 0.64 eV and 0.60 eV, respectively. Addition of four C
atoms causes an intermediate IP reduction (−0.27), while five and six C atoms cause
smaller IP reductions (−0.15 and −0.13 eV, respectively).



                                           113
Chapter 6: Tantalum-Carbide Clusters




                                                                                                                                               (a) : Ta
                                                                                                                                                          3
                                                                                                                                               IP = 5.59 eV




                                                                                                   Cluster Signal Intensity (Arb. Units)
                                                                                a) 262nm
                                                                                                                                               (b) : Ta C
                                                                                                                                                          3
                                                                                                                                               IP = 5.67 eV
Cluster Signal Intensity (Arb. Units)




                                                                                b) 245nm
                                                                                                                                               (c) : Ta C
                                                                                                                                                          3   2
                                                                                                                                               IP = 5.24 eV




                                                                                c) 230nm                                                       (d) : Ta C
                                                                                                                                                          3   3
                                                                                                                                               IP = 5.51 eV




                                                                                                                                           5              5.2         5.4   5.6    5.8   6
                                                                                d) 220nm                                                                          Photon Energy (eV)

                                                                                                   Figure 6-3: Photo-ionisation efficiency
                                                                                Ta C
                                                                                  5    2
                                                                                                   spectra for the Ta3Cy (y = 0–3)
                                                               Ta C
                                                                 4    2                            clusters. The determined IPs are also
                                              Ta C
                                                3    2                          e) 210nm
                                                                                                   displayed.
                                        500   600        700     800      900         1000
                                                         Mass (M/Z)

Figure 6-2: Mass spectra of Ta3Cy,
Ta4Cy and Ta5Cy clusters at four
different ionisation wavelengths:
(a) 262 nm, (b) 245 nm, (c) 230 nm,
(d) 220 nm and (e) 210 nm.




                                                                                             114
Chapter 6: Tantalum-Carbide Clusters



                                            (a) : Ta                                                                                    (a) : Ta
                                                       4                                                                                           5
                                            IP = 5.82 eV                                                                                IP = 5.41 eV




                                            (b) : Ta C                                                                                  (b) : Ta C
Cluster Signal Intensity (Arb. Units)




                                                       4                                                                                           5
                                            IP = 5.81 eV                                                                                IP = 5.44 eV




                                            (c) : Ta C                                                                                  (c) : Ta C
                                                       4   2                                                                                       5   2
                                            IP = 5.56 eV




                                                                                                Cluster Signal Intensity (Arb. Units)
                                                                                                                                        IP = 4.77 eV




                                            (d) : Ta C                                                                                  (d) : Ta C
                                                       4   3                                                                                       5   3
                                            IP = 5.77 eV                                                                                IP = 4.81 eV




                                            (e) : Ta C                                                                                  (e) : Ta C
                                                       4   4                                                                                       5   4
                                            IP = 5.14 eV                                                                                IP = 5.14 eV




                                        5              5.2         5.4   5.6    5.8   6                                                 (f) : Ta C
                                                                                                                                               5       5
                                                               Photon Energy (eV)                                                       IP = 5.26 eV


Figure 6-4: Photo-ionisation efficiency
spectra for the Ta4Cy (y = 0–4)
clusters. The determined IPs are also                                                                                                   (g) : Ta C
                                                                                                                                                   5   6
displayed.                                                                                                                              IP = 5.28 eV




                                                                                                                                         4.6           4.8     5   5.2   5.4    5.6   5.8
                                                                                                                                                           Photon Energy (eV)

                                                                                                Figure 6-5: Photo-ionisation efficiency
                                                                                                spectra for the Ta5Cy (y = 0–6)
                                                                                                clusters. The determined IPs are also
                                                                                                displayed.




                                                                                          115
Chapter 6: Tantalum-Carbide Clusters



6.2 Comparison between Calculated and Experimental IPs

For each tantalum-carbide cluster species DFT calculations are only performed on the
low energy structural isomers examined with the extended basis set for the analogous
niobium-carbide cluster (see Section 5.3). However, the tantalum-carbide cluster
calculations are only conducted with the SDD basis set, not the extended basis set.
Note that the isomer labels given (e.g. A, B, C, etc) reflect the energy ordering of the
isomers. Therefore, an assigned isomer label can refer to a different structural isomer
for a tantalum-carbide cluster species, compared to the niobium-carbide cluster
species, if structural isomer energy reordering occurs between the iso-valent clusters.


All the calculated isomers considered for the neutral TaxCy clusters are shown in
Figure 6-6 (Ta3Cy, y = 0–3), Figure 6-8 (Ta4Cy, y = 0–4), Figure 6-10 (Ta5Cy, y = 0–4)
and Figure 6-11 (Ta5Cy, y = 5 and 6). The relative energies (∆E in eV) for each isomer
of the neutral species are also given in the figures. It is important to note that similar
geometric minima for each isomer are also identified on the cationic surface. All
details (i.e. geometric and energy information) for both neutral and cationic isomers
examined are contained in Appendix IV and V.


Identical to the procedure for the niobium-carbide clusters (Section 5.3), adiabatic
ionisation transitions are considered for the lowest energy isomer, as well as the low-
lying isomers, for each tantalum-carbide cluster species. Similarly, only ionisation
energies including ZPE are considered for discussion. As observed for the NbxCy
clusters, the absolute calculated IPs for all the TaxCy isomers are higher than the
experimental values. Identical to the procedure for the NbxCy clusters, a linear offset is
applied to the IPs of the TaxCy isomers to generate an offset value, IP†. These
ionisation energies are listed for the Ta3Cy (y = 0–3), Ta4Cy (y = 0–4) and Ta5Cy (y =
0–6) clusters in Table 6-1, Table 6-2 and Table 6-3, respectively. In addition,
calculated IP†s for each isomer and experimental IPs for each of the Ta3Cy (y = 0–3),
Ta4Cy (y = 0–4) and Ta5Cy (y = 0–6) cluster series are also shown in Figure 6-7,
Figure 6-9 and Figure 6-12, respectively.




                                            116
Chapter 6: Tantalum-Carbide Clusters


6.2.1 The Ta3Cy (y = 0–3) Cluster Series

For the Ta3Cy cluster series a large offset of −0.611 eV is applied so that the
calculated and experimental IPs of Ta3 overlap. The resultant IP† is the value shown in
final column of Table 6-1, while Figure 6-7 shows the calculated IP†s of the isomers
( ) for each Ta3Cy species, as well as the experimental IP ( ).




Figure 6-6: Structures of calculated isomers for the neutral Ta3Cy (y = 0–3) clusters.
Written beneath each isomer are the relative energies (∆E in eV) calculated using the
SDD basis set.


For Ta3 an obtuse triangle of high spin multiplicity was found to be the lowest energy
structure (XX A [4A2, C2v]), consistent with the proposed non-equilateral triangular
structure from Raman studies of Ta3 deposited in an argon matrix.2

For Ta3C, isomer XXI A [2A′, Cs] has the C atom bound to a Ta face of the triangular
Ta3 cluster, whereas in isomer XXI B [2A1, C2v] the C atom is bound across a Ta-Ta
edge. Both isomers are essentially equal in energy (XXI B ∆E = +0.025 eV) but have
quite different calculated IP†s; XXI A has a deviation of −0.188 eV between its IP†


                                         117
Chapter 6: Tantalum-Carbide Clusters


and the experimental value, whereas XXI B has a larger deviation of +0.463 eV.
Therefore, isomer XXI A is assigned to the experimental IP.


                                                                      Calc.          Calc. IP     Calc. IP
                                                                                                                                              †
    Cluster                       Expt. IP           Isomer         Transition     (exc. ZPE)   (inc. ZPE)   Calc. IP

                                                                    3
      Ta3                          5.59                  XX A           B1 ← 4A2       6.197      6.201       5.590

                                                                    3
     Ta3C                          5.67              XXI A              A″ ← 2A′       6.084      6.093       5.482
                                                                    1        2
                                                     XXI B              A1 ← A1        6.734      6.744       6.133

                                                                    1
    Ta3C2                          5.24              XXII A         A1′ ← 2A′          5.748      5.767       5.156

                                                                    1
    Ta3C3                          5.51              XXIII A            A′ ← 2A′       6.293      6.318       5.707
                                                                        3    2
                                                     XXIII B            A← A           7.129      7.130       6.519


Table 6-1: List of experimental ionisation potentials (reported in eV) observed for
Ta3Cy (y = 0–3) clusters. Also listed are calculated transitions and ionisation
potentials: excluding ZPE, including ZPE, and offset IP (i.e. IP†).




                            6.5                                                                     B


                                                                                                                  Calculated Offset IP (eV)
     Experimental IP (eV)




                                                                B
                            6.0


                                                                                                    A

                            5.5                                 A
                                             Ta C Expt. IP
                                                 3   y



                                             Ta C Calc. IP
                                                 3   y
                                                                                   A

                            5.0
                                             0                    1            2                        3
                                                                No. Carbon atoms (y)
Figure 6-7: Graph showing experimental IP values for the Ta3Cy clusters as a
function of y. Also shown on the same scale are the offset values, IP†, calculated using
DFT. The letters (A, B, etc.) denote the isomers for that particular cluster (see text for
details).




                                                                             118
Chapter 6: Tantalum-Carbide Clusters


The only isomer considered for Ta3C2, XXII A [2A′, Cs], is that where both C atoms
are bound to opposite Ta faces of the triangular Ta3 cluster. Its calculated IP† is in
excellent agreement with the experimental value, with a deviation of −0.084 eV.


The isomer XXIII A [2A′, Cs] of Ta3C3 is much lower in energy than XXIII B [2A,
C1] (∆E = +0.833 eV) and also has a calculated IP† only 0.197 eV higher in energy
than the experimental IP and so is assigned as the carrier of the observed ionisation
onset. Isomer XXIII A has one C atom bound to a Ta face and the remaining two C
atoms bound across separate Ta-Ta edges of the triangular Ta3 cluster.


6.2.2 The Ta4Cy (y = 0–4) Clusters Series

For the Ta4Cy cluster series an offset of −0.320 eV is applied so that the calculated
and experimental IPs of Ta4 overlap. The resultant IP† is the value shown in final
column of Table 6-2. Figure 6-9 shows the calculated IP†s of the isomers ( ) for each
Ta4Cy species, as well as the experimental IP ( ).

The lowest energy structure for Ta4 is an ideal tetrahedron (XXIV A [1A1, Td]).

The two isomers of Ta4C have the C atom either bound to a Ta face (XXV A [1A′,
Cs]) or Ta-Ta edge (XXV B [1A1, C2v]) of the tetrahedral Ta4 cluster. As both isomers
calculated IP†s are in excellent agreement with the experimental value (0.039 and
0.068 eV higher in energy, respectively), XXV A is assigned to ionisation onset as it
is 0.281 eV lower in energy. This is not a definitive assignment though, due to the
small ∆E of XXV B.

The two isomers of Ta4C2 are separated by 0.534 eV; XXVI A [3A2, C2v] being lower
in energy than XXVI B [1A1, C2v]. Isomer XXVI A is assigned as the carrier of the
experimental IP as the deviation between its IP† and the experimental value (+0.143
eV) is much smaller than that of XXVI B (+0.401 eV). XXVI A has both C atoms
bound to separate Ta faces of the tetrahedral Ta4 cluster and has a high spin
multiplicity.




                                         119
Chapter 6: Tantalum-Carbide Clusters


The IP† of Ta4C3 XXVII A [1A1, C3v] is in excellent agreement with the experimental
IP (a deviation of −0.070 eV). This is the only isomer considered for Ta4C3 and has
the C atoms bound to three of the four available Ta faces of the tetrahedral Ta4
cluster; i.e. a C deficient 2×2×2 nanocrystal.




Figure 6-8: Structures of calculated isomers for the neutral Ta4Cy (y = 0–4) clusters.
Written beneath each isomer are the relative energies (∆E in eV) calculated using the
SDD basis set.


The 2×2×2 nanocrystal isomer of Ta4C4 XXVIII A [3A″, Cs] has a calculated IP† only
0.097 eV higher in energy than the experimental IP and is consequently assigned as
the carrier of the ionisation onset. This is consistent with the structure proposed for
Ta4C4 based on its IR-REMPI spectrum.3 Note that this isomer has a triplet electronic
state, which is lower in energy (0.686 eV) than its singlet state (see Appendix IV).



                                          120
Chapter 6: Tantalum-Carbide Clusters



                                                                         Calc.             Calc. IP     Calc. IP
                                                                                                                                                         †
   Cluster                        Expt. IP          Isomer             Transition        (exc. ZPE)   (inc. ZPE)        Calc. IP

                                                                        2
     Ta4                           5.82          XXIV A                    A1 ← 1A1        6.142        6.140            5.820

                                                                        2
    Ta4C                           5.81             XXV A                   A′ ← 1A′       6.172        6.169            5.849
                                                                        2        1
                                                    XXV B                   A′ ← A1        6.200        6.198            5.878

                                                                        2
    Ta4C2                          5.57          XXVI A                     A ← 3A2        6.034        6.043            5.723
                                                                        2        1
                                                 XXVI B                    B2 ← A1         6.303        6.291            5.971

                                                                        2
    Ta4C3                          5.77         XXVII A                    A1 ← 1A1        6.161        6.160            5.840

                                                                       2
    Ta4C4                          5.14         XXVIII A                B1 ← 3A″           5.543        5.557            5.237

Table 6-2: List of experimental ionisation potentials (reported in eV) observed for
Ta4Cy (y = 0–4) clusters. Also listed are calculated transitions and ionisation
potentials: excluding ZPE, including ZPE, and offset IP (i.e. IP†).




                            6.0
                                                                                     B
                                                              B



                                                                                                                             Calculated Offset IP (eV)
                                                              A                                A
     Experimental IP (eV)




                                                                                     A



                            5.5



                                              Ta C Expt. IP
                                                3   y                                                               A

                                              Ta C Calc. IP
                                                3   y




                            5.0
                                          0                       1            2           3                    4
                                                                      No. Carbon atoms (y)


Figure 6-9: Graph showing experimental IP values for the Ta4Cy clusters as a
function of y. Also shown on the same scale are the offset values, IP†, calculated using
DFT. The letters (A, B, etc.) denote the isomers for that particular cluster (see text for
details).




                                                                                 121
Chapter 6: Tantalum-Carbide Clusters



6.2.3 The Ta5Cy (y = 0–6) Cluster Series

For the Ta5Cy cluster series an offset of −0.216 eV is applied so that the calculated
and experimental IPs of Ta5 overlap. The resultant IP† is the value shown in final
column of Table 6-3, while Figure 6-12 shows the calculated IP†s of the isomers ( )
for each Ta5Cy species, as well as the experimental IP ( ).




Figure 6-10: Structures of calculated isomers for the neutral Ta5Cy (y = 0–4)
clusters. Written beneath each isomer are the relative energies (∆E in eV) calculated
using the SDD basis set.


                                         122
Chapter 6: Tantalum-Carbide Clusters




Figure 6-11: Structures of calculated isomers for the neutral Ta5Cy (y = 5 and 6)
clusters. Written beneath each isomer are the relative energies (∆E in eV) calculated
using the SDD basis set.



The lowest energy structure for Ta5 is a distorted trigonal bipyramid (XXIX A, [2B1,
C2v]).


The two Ta5C isomers XXX A [2A′, Cs] and XXX B [2A, C1] are very close in energy,
with the former being 0.109 eV lower in energy. XXX A has the C atom bound across
a Ta butterfly motif of the oblate trigonal bipyramid Ta5 cluster, while XXX B has the
C atom bound to a Ta face of the trigonal bipyramid Ta5 cluster. A definitive
assignment of the experimental IP is not possible as both the calculated IP†s of XXX
A and XXX B are in excellent agreement with the experimental value; deviations of
+0.105 and +0.120 eV, respectively. The lowest energy isomer (XXX A) is therefore
assigned to the ionisation onset.


For Ta5C2, isomer XXXI A [2B1, C2v] is 0.454 eV lower in energy than XXXI B [2A′,
Cs]. Isomer XXXI A has the two C atom bound across separate Ta butterfly motifs of
the oblate trigonal bipyramid Ta5 cluster and is assigned to the experimental IP as its


                                          123
Chapter 6: Tantalum-Carbide Clusters


calculated IP† is only 0.123 eV higher, whereas the IP† of XXXI B has a much higher
deviation (+0.518 eV).


                                      Calc.          Calc. IP     Calc. IP
                                                                                      †
    Cluster   Expt. IP   Isomer     Transition     (exc. ZPE)   (inc. ZPE)   Calc. IP

                                    3
     Ta5       5.41      XXIX A     A1′ ← 2B1        5.627        5.626       5.410

                                    1
    Ta5C       5.44      XXX A          A′ ← 2A′     5.753        5.761       5.545
                                        3    2
                         XXX B          A← A         5.772        5.776       5.560

                                    1
    Ta5C2      4.77      XXXI A         A1 ← 2B1     5.101        5.109       4.893
                                    1        2
                         XXXI B         A′ ← A′      5.500        5.504       5.288

                                    1
    Ta5C3      4.81      XXXII A        A1 ← 2A′     5.186        5.197       4.981

                                    3
    Ta5C4      5.14      XXXIII A       A′ ← 2A′     5.973        5.983       5.767
                                    1        2
                         XXXIII B       A′ ← A″      5.596        5.612       5.396
                                    1        2
                         XXXIII C       A1 ← B2      5.747        5.764       5.548
                                        1    2
                         XXXIII D       A← B         5.489        5.502       5.286

                                    1
    Ta5C5      5.26      XXXIV A        A′ ← 2A′     5.642        5.667       5.451
                                    3        2
                         XXXIV B        A′ ← A′      5.968        5.979       5.763
                                    1        2
                         XXXIV C        A′ ← A′      5.494        5.515       5.299
                                        1    2
                         XXXIV D        A← A         5.842        5.856       5.640

                                    1
    Ta5C6      5.28      XXXV A         A1 ← 2A1     5.704        5.727       5.511
                                    1        2
                         XXXV B         A1 ← B1      6.115        6.166       5.950


Table 6-3: List of experimental ionisation potentials (reported in eV) observed for
Ta5Cy (y = 0–6) clusters. Also listed are calculated transitions and ionisation
potentials: excluding ZPE, including ZPE, and offset IP (i.e. IP†).


The only isomer considered for Ta5C3, XXXII A [2A′, Cs], has a calculated IP† in
excellent agreement with the experimental IP (with a deviation of +0.171 eV). This
isomer has the C atoms bound to all three of the available Ta faces which share a
common axial Ta atom of the trigonal bipyramid Ta5 cluster. This isomer can also be
considered as a substituted 2×2×2 nanocrystal, where a C atom is replaced with a Ta
atom. This is consistent with the structure proposed for Ta5C3 based on its IR-REMPI
spectrum.3



                                            124
Chapter 6: Tantalum-Carbide Clusters



                            6.0
                                                                                     B


                                                                     A       B




                                                                                             Calculated Offset IP (eV)
     Experimental IP (eV)
                                                                             D
                                            B
                                                                     C
                            5.5             A                                        A
                                                                             A
                                                                     B
                                                         B           D       C



                            5.0                              A
                                      Ta C Expt. IP      A
                                        5   y

                                      Ta C Calc. IP
                                        5   y




                                  0             1      2       3      4          5       6
                                                      No. Carbon atoms (y)

Figure 6-12: Graph showing experimental IP values for the Ta5Cy clusters as a
function of y. Also shown on the same scale are the offset values, IP†, calculated using
DFT. The letters (A, B, etc.) denote the isomers for that particular cluster (see text for
details).


Four isomers are examined for Ta5C4; XXXIII A [2A′, Cs], XXXIII B [2A″, Cs],
XXXIII C [2B, C2] and XXXIII D [2A1, C2v]. The lowest energy isomer (XXXIII A)
has the C atoms bound to four of the Ta faces of the trigonal bipyramid Ta5 cluster,
where three of the Ta faces share a common axial Ta atom. However, this isomer does
not have a calculated IP† which agrees with the ionisation onset, being 0.627 eV
higher in energy than the experimental value. Of the remaining isomers only XXXIII
B and XXXIII D have IP†s in good agreement with the experimental IP (see Figure
6-12) with deviations of +0.256 and +0.146 eV, respectively. The observed ionisation
onset is assigned to XXXIII B, as it is much lower in energy than XXXIII D; ∆E =
+0.843 and +0.208 eV, respectively. The structure of XXXIII B can be considered as
a Ta atom bound above a C atom of a 2×2×2 nanocrystal.


Four isomers are also examined for Ta5C5; XXXIV A [2A′, Cs], XXXIV B [2A, C1],
XXXIV C [2A′, Cs] and XXXIV D [2B1, C2v]. Isomers XXXIV B and XXXIV D are
discounted as their IP†s are not in reasonable agreement with the experimental value;
deviations of +0.503 and +0.380 eV, respectively. Both isomers XXXIV A and


                                                             125
Chapter 6: Tantalum-Carbide Clusters


XXXIV C have calculated IP†s in excellent agreement with the experimental value
(see Figure 6-12), being only 0.191 and 0.39 eV higher in energy, respectively.
Therefore, the calculated IP†s cannot distinguish between XXXIV A, which has all C
atoms bound to separate Ta faces of the trigonal bipyramid Ta5 cluster, and XXXIV
C, which contains a C2 unit. As XXXIV A is calculated to be the lowest energy
isomer by 0.577 eV, it is assigned to the experimental ionisation onset.


The lowest energy isomer of Nb5C6 (XXXV A [2A1, C2v]) has an acetylide C2 unit
bound across each of the two Ta butterfly motifs of the oblate trigonal bipyramid Ta5
cluster, with the remaining two C atoms bound to the two outer Ta faces. Isomer
XXXV A is assigned to the ionisation onset as its calculated IP† is in much better
agreement with the experimental IP (a deviation of +0.231 eV) compared to the
higher energy isomer XXXV B, which has an IP deviation of +0.670 eV.




                                          126
Chapter 6: Tantalum-Carbide Clusters



6.3 Discussion on Tantalum-Carbide Clusters


6.3.1 Comparison to the Niobium-Carbide Clusters

The extracted IPs for the tantalum-carbide clusters in each series show trends that are
very similar to the iso-valent niobium-carbide cluster series, although the IP
reductions upon addition of C atoms are greater for the latter.


Furthermore, there are other differences to note between the tantalum-carbide and
niobium-carbide clusters. An ionisation onset is observed for Ta3C but not for Nb3C,
since it was beyond the range of our experiment (> 5.91 eV). This is attributed to the
fact that the in-plane structure (i.e. M-M edge bound C atom) is the global minimum
for Nb3C and has a much higher IP than the out-of-plane isomer (i.e. M face bound C
atom). Conversely, the lowest energy isomer of Ta3C is the out-of-plane structure.
Additionally, ionisation onsets are not observed for Ta3C4, Ta4C5 and Ta4C6. It is not
clear if these species are simply not generated under our experimental conditions or
whether there are other factors at play. For example, it may be that for Ta3C4 the low-
lying structural isomer assigned to the ionisation onset of Nb3C4, which has a C2 unit
and a much lower IP than the global minimum, is not generated. While for Ta4C5 and
Ta4C6 the higher spin states may not be metastable, as proposed for Nb4C5 and Nb4C6.


Apart from Ta5C and Ta5C5, all other TaxCy species identified have the same
structural isomer assigned to their ionisation onsets as those assigned for the
corresponding NbxCy species. However, for both these metal-carbide cluster species in
the Ta and Nb cases, two isomers are identified which both have IPs in excellent
agreement with the experimental values. Therefore, the assignments are made to the
lowest energy isomer, with the energy ordering of the two structural isomers
swapping between Ta and Nb.


In the example of M5C5 (for equivalent basis sets), the isomer containing a C2 unit is
significantly higher in energy relative to the isomer containing no C-C bonding for
Ta5C5, yet for Nb5C5 the two isomers are essentially equal in energy. Therefore, Ta-C
bonding appears to be slightly more energetically favorable than Nb-C bonding,


                                          127
Chapter 6: Tantalum-Carbide Clusters


relative to C-C bonding. This point is consistent with the fact that the Ta8C12 Metcar
(which is expected to contain 6 C2 units) has not been generated.4 This latter
observation was explained to be because of slightly stronger Ta-Ta bonding, relative
to Nb-Nb bonding; i.e. high C:Ta ratios for TaxCy clusters impose detrimental
disruptions to Ta-Ta bonding.5,6




                                         128
Chapter 6: Tantalum-Carbide Clusters



6.4 Summary

In summary, this chapter has shown that the isomeric structures of tantalum-carbide
clusters can be inferred by the determination of their IPs by PIE experiments in
combination with DFT calculations on candidate isomers.


The IP trends in each tantalum-carbide cluster series are very similar to the iso-valent
niobium-carbide cluster series, although the IP reductions upon addition of C atoms
are greater for the latter.


For the vast majority of tantalum-carbide cluster species, the same structural isomer is
assigned to the ionisation onset as those assigned for the corresponding niobium-
carbide cluster species. For the Ta3Cy and Ta4Cy cluster series these isomers
correspond to fragments of the 2×2×2 nanocrystal structure for Ta4C4. The only
cluster which has an isomer containing C-C bonding assigned to its ionisation onset is
Ta5C6, with the structure containing two C2 units.


The next chapter will describe the generation of novel bimetallic tantalum-zirconium-
carbide clusters, which are examined though essentially identical PIE experiments and
DFT calculations. The IP measurements and structural assignments made in this
chapter for the Ta4Cy clusters will serve as a comparison for the effects occurring
upon substitution of a Ta atom with a Zr atom to generate Ta3ZrCy clusters.




                                          129
Chapter 6: Tantalum-Carbide Clusters



6.5 References

        (1)     Collings, B. A.; Rayner, D. M.; Hackett, P. A. Int. J. Mass. Spec. Ion
Proc. 1993, 125, 207.
        (2)     Fang, L.; Shen, X.; Chen, X.; Lombardi, J. R. Chem. Phys. Lett. 2000,
332, 299.
        (3)     van Heijnsbergen, D.; Fielicke, A.; Meijer, G.; von Helden, G. Phys.
Rev. Lett. 2002, 89, 013401.
        (4)     Wei, S.; Guo, B. C.; Purnell, J.; Buzza, S.; Castleman Jr, A. W.
Science 1992, 256, 818.
        (5)     Cartier, S. F.; May, B. D.; Castleman Jr, A. W. J. Phys. Chem. 1996,
100, 8175.
        (6)     Armentrout, P. B.; Hales, D. A.; Lian, L. Collision-Induced
Dissociation of Transition-Metal Cluster Ions. In Cluster Reactions; Duncan, M. A.,
Ed.; Jai Press Inc.: Greenwich, CT, 1994; Vol. 2; pp 1.




                                         130
Chapter 7: Tantalum-Zirconium-Carbide Clusters



Chapter 7: Tantalum-Zirconium-Carbide Clusters

The following chapter is concerned with the development of a double ablation cluster
source to generate bimetallic-carbide clusters of suitable composition for experimental
PIE studies. As will be shown, mixing of tantalum (Group 5) and zirconium (Group 4)
transition metals to generate bimetallic-carbide clusters is achieved with this cluster
source. Bimetallic-carbide clusters containing three tantalum atoms and one
zirconium atom (i.e. Ta3ZrCy) are investigated through a combination of PIE
experiments and DFT calculations on their energetically favourable isomers.



7.1 Introduction to Bimetallic-Carbide Clusters

Bimetallic-carbide clusters are interesting systems to study because not only their
size, but also their composition, can be systematically varied. Constructing bimetallic-
carbide clusters of suitable composition may tailor new cluster properties, which are
not possessed by any monometallic-carbide clusters. Extensive studies have been
performed on the properties and reactivity of bare bimetallic clusters; notably by the
separate research groups of Kaya,1-3 Knickelbein,4,5 Silverans and Lievens,6-8 and
Wang.9,10


However, experimental studies on bimetallic-carbide clusters have been limited to
those performed by Castleman and co-workers on the Metcar species. Their initial
studies focused on the generation of the titanium Metcar, where titanium atoms were
substituted with various alternative metal atoms; i.e. Ti8−xMxC12, where M = Zr, Hf,
Y, Nb, Mo, Ta or W.11,12 Zirconium (which is in the same group as titanium)
displayed the greatest degree of substitution, with up to five atoms incorporated into
the bimetallic Metcar. Further studies examined the delayed ionisation and delayed
atomic ion emission of the Ti8−xMxC12 clusters, where M = Zr or Nb and x = 0–4,13
and also determined the IPs by PIE experiments of the Ti8−xZrxC12 clusters (where x =
0–4 and 8).14 These latter experiments showed that the IP of the titanium Metcar (4.40
eV) is higher than that of the zirconium Metcar (3.95 eV), with the IP of the
bimetallic Metcar clusters decreasing continuously from the pure titanium Metcar



                                          131
Chapter 7: Tantalum-Zirconium-Carbide Clusters


towards that of the pure zirconium Metcar, as the number of substituting zirconium
atoms increased.


Poblet, Rohmer, Benard and co-workers used DFT to investigate the bimetallic
Metcar clusters generated experimentally by Castleman and co-workers.15 For the
Ti8−xZrxC12 (x = 0–5) clusters, two isomers were considered for each stoichiometry;
i.e. the metal with the lowest number of atoms present would have all its atoms
located in either the inner tetrahedron positions or the larger outer tetrahedron
positions, but never a combination of the two. The difference in energy between the
two isomers for each cluster was found to be very small (never exceeding 0.065 eV).
The similar energy for substitution in either of the two different metallic positions in
the titanium Metcar explained the regular statistical distribution of the peaks in the
mass spectrum of Ti8−xZrxC12 (x = 0–5), which had initially been interpreted as
evidence that all eight of the metal atoms in Ti8C12 were located in symmetry
equivalent positions.11


Of interest to the present study are investigations performed on metal-oxide cluster
anions. IR-MPD spectra were obtained for vanadium-oxide cluster anions of the form
(V2O5)x−.16 In comparison with DFT calculations, the structures were shown to be
based on polyhedral cages. Further studies performed on bimetallic vanadium-
titanium-oxide cluster anions of stoichiometry (V2O5)x(VTiO5)− (x = 1–3) showed that
they also have the same structures as the corresponding pure vanadium-oxide cluster
anions and those predicted for the iso-electronic neutral vanadium-oxide clusters.17
Additional investigations on the V4−xTixO10− (x = 1–4) clusters showed that these
species also possessed the same polyhedral caged structure as V4O10−. Overall this
work showed that substitution of a Group 5 transition metal atom with a Group 4
transition metal atom does not change the structure of the metal-oxide clusters in this
size range. This is relevant to the current study as a similar substitution regime is
performed, albeit for metal-carbide clusters.




                                          132
Chapter 7: Tantalum-Zirconium-Carbide Clusters



7.2 Double Ablation Cluster Source Design

Bimetallic clusters can be generated by either ablation of an alloy target with varying
molar composition or by mixing the ablated products from two separate metal targets.
The ablation of separate metal targets is the approach chosen for our double ablation
cluster source. This technique is much more flexible as any combination of elements
can in principle be used and the power and timing of the separate ablation lasers can
be varied individually to optimise generation of the desired bimetallic cluster.


The double ablation source design (Figure 7-1) is similar to that originally proposed
by Kaya and co-workers1 and is based on our previous single ablation source (see
Section 2.1.2), except that two 2 mm diameter rods are utilised instead of one 5 mm
diameter rod. In the double ablation cap the two metal rods are displaced by a distance
of 5 mm along the gas channel. The two rods are situated on opposite sides of the gas
channel. Therefore, the two rods are ablated from opposite sides of the ablation cap.
Both rods in the double ablation source are rotated and translated by the same screw
mechanism motor in conjunction with a gear mechanism. The gas channel in the
double ablation cap has a 3 mm internal diameter, increased from the 2 mm diameter
in the single ablation setup.




Figure 7-1: Double ablation source used to generate bimetallic-carbide clusters
shown in the (a) side-on and (b) isometric orientation. Dimensions given in mm.




                                          133
Chapter 7: Tantalum-Zirconium-Carbide Clusters


The condensation tube (10 mm in length) is also slightly modified, with a screw
thread tapped into the end of the inner channel so that further tube attachments of
alternating lengths can be connected and interchanged easily. The internal diameter of
the double ablation condensation tube and attachments is maintained at 2 mm. In the
current experiment a condensation tube attachment of 5 mm is connected, making the
total condensation tube length 15 mm (identical to that in the single ablation
experiments).


To improve the amount of generated clusters reaching the ionisation region the length
of the hollow extension cylinder was increased to 15.2 cm, making the output of the
condensation tube now only ~10 cm away from the skimmer.


The two rods are ablated with focussed 532 nm laser pulses originating from separate
Nd:YAG laser systems. The relative timing of each ablation laser is controlled with
the digital pulse generator, with the actual separation time between ablation events
confirmed with a fast photodiode.




                                         134
Chapter 7: Tantalum-Zirconium-Carbide Clusters



7.3 Generation of Tantalum-Zirconium-Carbide Clusters

Bimetallic-carbide clusters are chosen to be constructed from tantalum (Group 5) and
zirconium (Group 4) transition metals. Generating tantalum-zirconium-carbide
clusters containing one or more Zr atoms, with a total number of 3–5 metal atoms, are
desired as the IPs and structures of tantalum-carbide clusters containing 3–5 tantalum
atoms were determined in Chapter 6.


The primary difficulty in generating bimetallic-carbide clusters composed of Ta and
Zr atoms is that the monometallic-carbide clusters may overlap in mass with the
bimetallic-carbide clusters, rendering the PIE technique unable to be applied. This
overlap is primarily because the mass of a Zr atom (isotope pattern: 90Zr 51.45%, 91Zr
11.22%, 92Zr 17.15%, 94Zr 17.38% and 96Zr 2.80 %) is approximately half that of a Ta
atom (181Ta 99.99%). For example, TaxZrzCy clusters containing one Zr atom will
overlap with ZrxCy clusters containing an odd number of Zr atoms, while TaxZrzCy
clusters containing two Zr atoms will overlap with ZrxCy clusters containing an even
number of Zr atoms and TaxCy clusters. This overlap is likely to be occur under SPI
conditions as the ZrxCy and TaxZrzCy clusters are expected to have IPs in a similar
energy range to that of the TaxCy clusters, as the IPs of the Zr atom (6.63 eV) and Zr3
cluster (5.22 eV)18 are lower than that of the corresponding Ta atom (7.89 eV) and Ta3
cluster (5.59 eV), respectively.


In consideration of the desired bimetallic-carbide clusters to be generated and the
above points, tantalum-zirconium-carbide clusters which only contain one Zr atom are
the most practical candidates, as the only source of mass interference will be large
zirconium-carbide clusters containing an odd number of Zr atoms (e.g. the Zr7Cy
clusters will interfere with the Ta3ZrCy clusters).


Figure 7-2 shows the mass spectrum of ZrxCy clusters generated from the ablation of
the Zr rod, under essentially identical conditions to those described for generation of
NbxCy and TaxCy clusters (see Section 2.1.2), although now a slightly higher ablation
power is used (~8 mJ). This mass spectrum is obtained with both the Zr and Ta rods
in the double ablation source, yet only the Zr rod is ablated. The mass distribution is


                                           135
Chapter 7: Tantalum-Zirconium-Carbide Clusters


similar to that observed for the NbxCy and TaxCy clusters (see Sections 5.2 and 6.1),
although the present spectrum is significantly congested due to the isotope pattern of
the Zr atom. Clusters containing oxygen are also present due to an oxide layer on the
Zr rod. Overall from this mass spectrum it can be seen that large ZrxCy clusters are
readily generated and may overlap with the desired TaxZrCy clusters.
      Intensity (Arb. Units)




                                   ZrC
                                         y
                                                                                                                                 Zr C
                                                                                                                                  10    y


                                             Zr C                                                                       Zr C
                                              2     y                                                                    9   y

                                                                                                             Zr C
                                                                                                              8     y


                                                                                                  Zr C
                                                                                                   7     y
                                                        Zr C                             Zr C
                                                         3     y                          6   y
                                                                   Zr C
                                                                    4     y
                                                                              Zr C
                                                                               5     y




                               0             200                   400                   600                  800                 1000      1200   1400   1600
                                                                                                   Mass (M/Z)

Figure 7-2: Mass spectrum of ionised neutral ZrxCy clusters obtained by single-
photon ionisation (220 nm).



Initial generation of TaxZrCy clusters is approached with the Ta rod in the first
position (i.e. closest to the pulsed nozzle) and the Zr rod in the second position of the
double ablation cap. As Ta rich TaxZrzCy clusters are desired, the Ta rod is ablated
first (~ 6 mJ), on the leading edge of the gas pulse. The Zr rod, located 5 mm away, is
then ablated after a time delay with a lower laser power (~5 mJ) than applied to obtain
the above mass spectrum of ZrxCy clusters. This time delay (~ 6 µs) is employed so
that the ablated Ta and Zr products (the former being more abundant) are overlapped
to facilitate mixing.


Figure 7-3 shows the mass spectrum of the clusters generated by the procedure
described above. Ionisation is conducted at 220 nm, under SPI conditions. As can be
seen it is very similar in appearance to the addition of the two mass spectra of the
separately ablated Zr and Ta (see Section 6.1) rods. A threshold ablation power (~5
mJ) is found for the ablation of the Zr rod, with large ZrxCy clusters rapidly appearing



                                                                                                             136
Chapter 7: Tantalum-Zirconium-Carbide Clusters


above this power, which makes controlling the concentration of Zr products difficult.
Any TaxZrCy clusters generated by this procedure are not produced in significant
concentration, as blocking the Ta ablation laser has no significant effect on peaks
present in their expected mass regions, demonstrating that these peaks are primarily
ZrxCy clusters.


However, TaxZrCy clusters are generated in substantial concentration, without having
interference from large ZrxCy clusters, by a procedure of changing the relative timing
and power of the ablation lasers. Here the Zr rod is ablated ~7 µs earlier than the Ta
rod, with both rods ablated with ~ 6 mJ and the pulsed nozzle and Ta ablation laser
still triggered at the same relative time as before. By applying this procedure it is
found that the distribution of the clusters differs at various regions in the molecular
beam. These different regions are investigated by changing the timing of the
ionisation laser. Ionisation is similarly conducted at 220 nm, under SPI conditions.


Parts a–e of Figure 7-4 show mass spectra recorded at 10 µs firing intervals of the
ionisation laser. Time zero is arbitrarily set to that used when recording PIE spectra.
The mass spectrum at −30 µs (Figure 7-4a) primarily consists of only ZrxCy clusters.
The next mass spectrum at −20 µs (Figure 7-4b) also primarily consists of ZrxCy
clusters, but now a new large peak appears at mass 567, due to Ta3C2. Next at −10 µs
(Figure 7-4c) there is a decrease in ZrxCy clusters but an increase in TaxCy clusters
observed. Furthermore, the peaks in the mass region of the TaxZrCy (x = 3–5) cluster
have become sharper, indicating that these peaks are now bimetallic-carbide clusters
rather than ZrxCy clusters, which are broader due to the many combinations of the Zr
atom isotopes. The next mass spectrum at 0 µs (Figure 7-4d) primarily consists of
TaxCy clusters and TaxZrCy (x = 3–5) clusters. Interestingly, bimetallic-carbide
clusters of the form Ta2ZrCy are not observed with any great intensity, possibly as
their IPs are greater than the photon energy available at 220 nm. Negligible amounts
of ZrxCy clusters are now present, as evident by the absence of substantial low mass
peaks. An expanded larger version of this 0 µs mass spectrum is also shown in (Figure
7-6). The last mass spectrum at +10 µs (Figure 7-4e) primarily consists of only TaxCy
clusters, with small amounts of TaxZrCy clusters still present.




                                          137
Chapter 7: Tantalum-Zirconium-Carbide Clusters



                                                                                                                                                                         (a) −30µs, (b) −20µs, (c) −10µs, (d)
Intensity (Arb. Units)


                                                                                                               Zr C
                                                                                   Zr 7Cy                              9       y

                                                                                                                       Ta C / Zr C
                                                                                                                               5       y       10   y
                                                                                                                                                                         0µs and (e) +10µs.

                                                                                       Ta C / Zr C
                                                                                           4       y           8       y



                                                                     Ta C / Zr C
                                                                       3   y       6   y




                                             0         200    400          600                     800                                 1000                 1200
                                                                     Mass (M/Z)

Figure 7-3: Mass spectrum of ionised
neutral clusters generated by the




                                                                                                                                                                          Cluster Signal Intensity (Arb. Units)
ablation of the Zr rod 6µs latter than
                                                                                                                                                                                                                      a) Ta ablation only
the Ta rod (see text).



                                                                                                                   Zr C
                                                                                                                           9       y
                                                                                       Zr C
                                                                                           7   y

                                                                                                                                                                                                                      a) Zr ablation only



                                                                                                                                                                                                                                                           Ta ZrC     Ta ZrC
                                                                                                                                       a) − 30 µs                                                                                             Ta ZrC
                                                                                                                                                                                                                                                  3   y
                                                                                                                                                                                                                                                             4   y      5   y




                                                                                                                                                                                                                      c) Ta & Zr ablation
           Cluster Signal Intensity (Arb. Units)




                                                                                                                                                                                                                  0        200      400     600           800        1000       1200
                                                                                                                                                                                                                                      Mass (M/Z)
                                                                                                                                       b) − 20 µs
                                                                                                                                                                         Figure 7-5: Mass spectra of ionised
                                                                                                                                                                         neutral clusters generated by the
                                                                                                                                                                         ablation of the Zr rod 7µs earlier than
                                                                                                                                                                         the Ta rod (see text).The spectra are
                                                                                                                                                                         collected at the 0µs ionisation time.
                                                                                                                                       c) − 10 µs
                                                                                                                                                                         Clusters generated when (a) only the
                                                                                                                                                                         Ta rod is ablated, when (b) only the Zr
                                                                                                           Ta ZrC                          Ta ZrC
                                                                                   Ta ZrC
                                                                                       3       y
                                                                                                                   4               y           5        y
                                                                                                                                                                         rod is ablated and when (c) both rods
                                                                                                                                                                         are ablated together are shown.

                                                                                                                                       d) 0 µs



                                                                                                                                   Ta C
                                                                                                                                       5   y




                                                                                               Ta C
                                                                                                       4   y



                                                                           Ta C
                                                                               3   y

                                                                                                                                       e) + 10 µs

                                                   0    200    400             600                     800                             1000                 1200
                                                                    Mass (M/Z)
Figure 7-4: Mass spectra of ionised
neutral clusters generated by the
ablation of the Zr rod 7µs earlier than
the Ta rod (see text).The spectra differ
by the timing of the ionisation laser:



                                                                                                                                                                   138
Chapter 7: Tantalum-Zirconium-Carbide Clusters




                                                                                                                       Ta ZrC               Ta C
                                                                                                      Ta ZrC               7    y              9   y
                                                                                                          6    y
     Intensity (Arb. Units)       TaC
                                                                     Ta ZrC
                                                                         4    y
                                                                                      Ta ZrC
                                                                                          5   y
                                                                                                                               Ta C
                                    y               Ta ZrC                                                                          8   y
                                                         3   y
                                                                                                              Ta C
                                                                                                                   7   y


                                                                                              Ta C
                                                                                                  6   y



                                                                             Ta C
                                                                                  5   y




                                                             Ta C
                                                                 4   y



                                              Ta C
                                                3    y




                              0   200   400   600                800              1000            1200                 1400             1600           1800
                                                                 Mass (M/Z)

Figure 7-6: Mass spectrum of ionised neutral clusters generated by the ablation of
the Zr rod 7µs earlier than the Ta rod (see text).The spectrum is collected at the 0µs
ionisation time and by single-photon ionisation (220 nm).



Although the mass assignment and the widths of the peaks assigned to the TaxZrCy
clusters (x = 3–5) are consistent with these species, a further check that these peaks
are indeed bimetallic-carbide clusters is to stop ablating each metal rod separately at
the 0 µs ionisation laser time. Parts a–c of Figure 7-5 show the mass spectra obtained
when only either the Ta or Zr rod is ablated and when both rods are ablated. Ablation
of only the Ta rod (Figure 7-5a) generates TaxCy clusters and no peaks in the region of
the TaxZrCy clusters, as expected. Conversely, ablation of only the Zr rod (Figure
7-5b) generates very weak amounts of ZrxCy (x = 1–2) clusters and no peaks in the
region of the TaxZrCy clusters. This indicates that indeed the TaxZrCy clusters are
generated, not ZrxCy clusters. Furthermore, this suggests that in this region of the
molecular beam the majority of ablated Zr products are present as ZrCy (x = 1–2)
species, which are not ionised at the photon energy used.


It is not immediately apparent as to why the first procedure (i.e. Ta ablated before Zr)
does not generate TaxZrCy clusters (or alternatively generates TaxZrCy clusters
overlapped with large ZrxCy clusters), whereas the second procedure (i.e. Ta ablated
after Zr) does generate TaxZrCy clusters and not large ZrxCy clusters (which would
overlap with the former).




                                                                         139
Chapter 7: Tantalum-Zirconium-Carbide Clusters


In the first procedure what is thought to be occurring is that the Ta rod is ablated on
the leading edge of the gas pulse (i.e. He seeded with C2H2), which carries the ablated
products towards the Zr rod, with initial TaxCy cluster generation occurring in this
time. When the Zr rod is ablated, the majority of previously generated TaxCy clusters
are dissociated, creating an abundance of Ta atoms and also (due to the threshold
ablation power) an abundance of Zr atoms. This generates large amounts of TaxCy and
ZrxCy clusters, and a small amount of TaxZrCy clusters. Furthermore, this procedure
leads to an essentially uniform distribution of generated clusters in the molecular
beam.


In the second procedure what is thought to be occurring is that the Zr rod is ablated
first before the leading edge of the gas pulse has reached this area. The Zr ablation
products therefore expand in both directions of the gas channel. The Ta rod is then
ablated ~ 7 µs later on the leading edge of the gas pulse. As the ablated Ta products
are carried downstream, generating initial TaxCy clusters as this occurs, they encounter
and overlap a small portion of the ablated Zr products which are travelling in the
opposite direction and, due to the absence of abundant bath gas, are still mainly
atomic species. There will be a region in this overlap area where the Ta atoms and
TaxCy clusters are in much greater concentration than the Zr atoms, so as that TaxCy
and TaxZrCy clusters are primarily generated, rather than large ZrxCy clusters. By
adjusting the timing of the ionisation laser this specific region of the molecular beam
can be investigated.




                                          140
Chapter 7: Tantalum-Zirconium-Carbide Clusters



7.4 Photo-ionisation Efficiency Experiments

All PIE spectra are collected for the Ta3ZrCy (y = 0–4) clusters with the Zr rod ablated
7 µs earlier than the Ta rod (i.e. the second ablation procedure) and the 0 µs laser
ionisation time.


Parts a–e of Figure 7-7 show a portion of the mass spectra of clusters generated from
the double ablation source following ionisation at five different wavelengths; 280,
265, 235 and 210 nm, collected under otherwise identical conditions. In the spectrum
recorded at 210 nm (Figure 7-7e), clusters containing Ta3Zr appear with 0–4 C atoms
attached. All of these species are produced following SPI.


The spectrum recorded at 280 nm (Figure 7-7a) shows only baseline levels, as none of
the species are ionised with one photon. At 265 nm (Figure 7-7b) Ta3ZrC4 is ionised,
the first bimetallic-carbide cluster to appear. Ionisation at 235 nm (Figure 7-7c) results
in Ta3ZrC2 appearing, followed by Ta3ZrC3 at 220 nm (Figure 7-7d). Finally back at
210 nm, Ta3Zr and Ta3ZrC are now ionised.


PIE spectra are recorded by monitoring the signal of each cluster as a function of
photon energy. PIE spectra for the Ta3ZrCy (y = 0–4) clusters are shown in parts a–d
of Figure 7-8. The cluster Ta3ZrC2 has a slightly steeper rising slope from its baseline
level, relative to the other clusters. For all spectra two lines are fitted; one to the
baseline and one to the linear rise of signal, and their intersection defined as the IP
(with an estimated error of ±0.05 eV). This procedure was previously described in
Section 2.2.4. The determined IPs for all the tantalum-zirconium-carbide clusters
considered in this study are displayed in the PIE spectra and are also given later in the
second column of Table 7-1.


For the Ta3ZrCy series, addition of one C atom only reduces the IP very slightly
relative to the IP of Ta3Zr, with a reduction of 0.05 eV. Addition of two and three C
atoms cause large (−0.64 eV) and intermediate (−0.37 eV) IP reductions, respectively.
A substantial IP reduction of 1.06 eV is observed for addition of four C atoms.




                                           141
Chapter 7: Tantalum-Zirconium-Carbide Clusters




                                                                                                                                            (a) : Ta Zr
                                                                                                                                                       3
                                                                                                                                                IP = 5.72 eV




                                                                            a) 270nm
                                                                                                                                            (b) : Ta ZrC
                                                                                                                                                       3




                                                                                                  Cluster Signal Intensity (Arb. Units)
                                                                                                                                                IP = 5.67 eV
Cluster Signal Intensity (Arb. Units)




                                                                            b) 262nm
                                                                                                                                            (c) : Ta ZrC
                                                                                                                                                       3       2
                                                                                                                                                IP = 5.08 eV




                                                                            c) 235nm                                                        (d) : Ta ZrC
                                                                                                                                                       3       3
                                                                                                                                                IP = 5.35 eV




                                                                                                                                            (e) : Ta ZrC
                                                                                                                                                       3       4
                                                                            d) 220nm                                                            IP = 4.66 eV




                                                             Ta ZrC
                                                                  3   2




                                                                            Ta C                                                          4.5                      5       5.5      6
                                                                              4   2
                                                                                                                                                               Photon Energy (eV)
                                              Ta C                          e) 210nm
                                                3    2

                                        550          600    650
                                                           Mass (M/Z)
                                                                      700   750       800         Figure 7-8: Photo-ionisation efficiency
                                                                                                  spectra for the Ta3ZrCy (y = 0–4)
Figure 7-7: Mass spectra of Ta3Cy,                                                                clusters. The determined IPs are also
Ta3ZrCy and Ta4Cy clusters at four                                                                displayed.
different ionisation wavelengths:
(a) 270 nm, (b) 262 nm, (c) 235 nm,
(d) 220 nm and (e) 210 nm.




                                                                                            142
Chapter 7: Tantalum-Zirconium-Carbide Clusters




7.5 DFT Calculated Isomers

For each tantalum-zirconium-carbide cluster species DFT calculations are only
performed on all possible isomers generated by substitution of a Ta atom with a Zr
atom for the lowest energy isomer of the corresponding tantalum-carbide cluster
species. All of the lowest energy isomers for the Ta4Cy clusters are assigned to the
experimental ionisation onsets (see Section 6.2.2). This procedure is considered
appropriate as only one Ta atom is substituted. Also, all Ta4Cy clusters had the same
structural isomer being the lowest in energy for both the neutral and cationic states,
with the latter being iso-electronic with the neutral Ta3ZrCy clusters. Furthermore, a
previous study has shown that the structures of bimetallic vanadium-titanium-oxide
clusters are the same compared to that of the corresponding vanadium-oxide
clusters.17


Calculations for the bimetallic-carbide clusters are conducted only with the SDD basis
set. Note that the isomer labels given (i.e. A and B) reflect the energy ordering of the
isomers, not specific structural isomers related to the tantalum-carbide clusters.


All the calculated isomers considered for the neutral Ta3ZrCy (y = 0–4) clusters are
shown in Figure 7-9. The relative energies (∆E in eV) for each isomer of the neutral
species are also given in the figure. Similar geometric minima for each isomer are also
identified on the cationic surface. All details (i.e. geometric and energy information)
for both the neutral and cationic isomers are contained in Appendix VI and VII.



7.5.1 The Ta3ZrCy (y = 0–4) Cluster Series

Recent DFT calculations have shown that Zr4 has a slightly distorted tetrahedral
geometry of high spin [7A′, Cs],19 while our calculations on Ta4 give an ideal
tetrahedron of low spin [1A1, Td]. A distorted tetrahedron (XXXVI A [2A′, Cs]) is
found to be the lowest energy structure for Ta3Zr. Note the symmetry of Ta3Zr
XXXVI A is Cs rather than the ideal C3v symmetry, consistent with Ta4+ being C2v



                                          143
Chapter 7: Tantalum-Zirconium-Carbide Clusters


rather than Td symmetry due to Jahn-Teller distortion. Of the four metal faces
available on Ta3Zr XXXVI A, there are three metal faces containing two Ta atoms
and one Zr atom (denoted “Ta-Zr face”) with the one remaining metal face containing
all Ta atoms (denoted “Ta face”).




Figure 7-9: Structures of calculated isomers for the neutral Ta3ZrCy (y = 0–4)
clusters. Written beneath each isomer are the relative energies (∆E in eV) calculated
using the SDD basis set.



Two isomers are possible for Ta3ZrC based on the structure of Ta4C XXV A; either
with the C atom bound to a Ta-Zr face (XXXVII A [2A, C1]) or the Ta face (XXXVII
B [2A′, Cs]) of the tetrahedral Ta3Zr cluster. The former isomer is the lowest in energy
by 0.222 eV.



                                          144
Chapter 7: Tantalum-Zirconium-Carbide Clusters



Two isomers are possible for Ta3ZrC2 based on the structure of Ta4C2 XXVI A. The
isomer where the two C atoms are bound to separate Ta-Zr faces of the tetrahedral
Ta3Zr cluster (XXXVIII A [2A, C1]) is the lowest in energy. However, the isomer
where one C atoms is bound to a Ta-Zr face and the remaining C atom to the Ta face
(XXXVIII B [2A″, Cs]) is essentially equal in energy (∆E = +0.024 eV).


Similarly two isomers are possible for Ta3ZrC3 based on the structure of Ta4C3
XXVII A; either all C atoms bound separately to all three available Ta-Zr faces of the
tetrahedral Ta3Zr cluster (XXXIX A [2A′, Cs]), or two C atoms bound to two Ta-Zr
faces and the remaining C atom bound to the Ta face (XXXIX B [2A″, Cs]). The
former is the lowest energy isomer by 0.132 eV.


Only one isomer for Ta3ZrC4 (XXXX A [2A′, Cs]) is possible by substituting a Ta
atom with a Zr atom in the 2×2×2 nanocrystal isomer of Ta4C4 XXVIII A; i.e. C
atoms are bound to all the available metal faces (three Ta-Zr faces and one Ta face)
on the tetrahedral Ta3Zr cluster. Note that XXXX A has Cs, rather than the ideal C3v
symmetry. Indeed, Ta4C4+ has a D2d symmetry, rather than an ideal Td symmetry.




                                         145
Chapter 7: Tantalum-Zirconium-Carbide Clusters



7.6 Comparison between Calculated and Experimental IPs

Identical to the procedure for the niobium-carbide and tantalum-carbide clusters
(Sections 5.3 and 6.2), adiabatic ionisation transitions are considered for the lowest
energy isomer, as well as the low-lying isomers, for each tantalum-zirconium-carbide
cluster species. Similarly, only ionisation energies including ZPE are considered for
discussion. These ionisation energies are listed for the Ta3ZrCy (y = 0–4) clusters in
Table 7-1. A small linear offset of −0.120 eV is applied to the IPs of the Ta3ZrCy
isomers, so that the calculated and experimental IPs of Ta3Zr coincide, to generate an
offset value IP†. The resultant IP† is the value shown in final column of Table 7-1.
Figure 7-10 shows the calculated IP†s of the isomers ( ) for each Ta3ZrCy species, as
well as the experimental IP ( ).


The calculated IP†s of the Ta3ZrC isomers XXXVII A and XXXVII B are both quite
close to the experimental valve, with deviations of −0.111 and +0.056 eV,
respectively. Therefore, the experimental IP is assigned to XXXVII A as it is lowest
energy isomer. However, this is not a definitive assignment as XXXVII B is only
0.222 eV higher in energy.


The two isomers of Ta3ZrC2, XXXVIII A and XXXVIII B, are essentially equal in
energy, with the former being the 0.024 eV lower in energy. However, the lowest
energy isomer (XXXVIII A) is assigned to the ionisation onset as its calculated IP† is
in better agreement with the experimental value than that of the low-lying isomer;
deviations of +0.316 and +0.562 eV, respectively.


The deviations between the calculated IP†s and experimental value for the XXXIX A
and XXXIX B isomers of Ta3ZrC3 are +0.507 and +0.272 eV, respectively. This
deviation for the lowest energy isomer (XXXIX A) is believed to be on the borderline
of what is considered to be an acceptable deviation between the calculated IP† and
experimental IP. As the low-lying isomer XXXIX B has an IP† in good agreement
with the experimental value and is only 0.132 eV higher in energy than XXXIX A, it
is assigned to the ionisation onset.




                                         146
Chapter 7: Tantalum-Zirconium-Carbide Clusters



                                                                      Calc.             Calc. IP     Calc. IP
                                                                                                                                                 †
 Cluster                      Expt. IP         Isomer               Transition        (exc. ZPE)   (inc. ZPE)    Calc. IP

                                                                     3
  Ta3Zr                           5.72        XXXVI A                    A″ ← 2A′       5.832        5.840        5.720

                                                                     1
 Ta3ZrC                           5.67       XXXVII A                    A′ ← 2A        5.659        5.679        5.559
                                                                     1        2
                                             XXXVII B                    A′ ← A′        5.844        5.846        5.726

                                                                     1
 Ta3ZrC2                          5.08       XXXVIII A                   A′ ← 2A        5.493        5.516        5.396
                                                                     1        2
                                             XXXVIII B                   A′ ← A″        5.747        5.762        5.642

                                                                     3
 Ta3ZrC3                          5.35        XXXIX A                    A′ ← 2A′       5.974        5.977        5.857
                                                                     3        2
                                              XXXIX B                A″ ← A″            5.731        5.742        5.622

                                                                     1
 Ta3ZrC4                          4.66        XXXX A                     A″ ← 2A′       5.145        5.167        5.047


Table 7-1: List of experimental ionisation potentials (reported in eV) observed for
Ta3ZrCy (y = 0–4) clusters. Also listed are calculated transitions and ionisation
potentials: excluding ZPE, including ZPE, and offset IP (i.e. IP†).



                            6.0

                                                                                            A
                                                           B

                                                                                                                     Calculated Offset IP (eV)
                                                                                  B         B
     Experimental IP (eV)




                                                           A
                            5.5
                                                                                  A




                                                                                                         A
                            5.0
                                             Ta ZrC Expt. IP
                                               3   y




                                             Ta ZrC Calc. IP
                                               3   y




                            4.5
                                         0                     1            2           3                    4
                                                                   No. Carbon atoms (y)
Figure 7-10: Graph showing experimental IP values for the Ta3ZrCy clusters as a
function of y. Also shown on the same scale are the offset values, IP†, calculated using
DFT. The letters (A and B) denote the isomers for that particular cluster (see text for
details).




                                                                              147
Chapter 7: Tantalum-Zirconium-Carbide Clusters


Only the substituted 2×2×2 cubic isomer (XXXX A) is examined for Ta3ZrC4, with it
being assigned to the experimental IP as its calculated IP† is in good agreement (a
deviation of +0.387 eV).




                                       148
Chapter 7: Tantalum-Zirconium-Carbide Clusters



7.7 Discussion on Tantalum-Zirconium-Carbide Clusters


7.7.1 Relative Energetics of the Bimetallic-Carbide Cluster Isomers

Substitution of a Ta atom with a Zr atom in the Ta4Cy clusters examined here shows
that substitution is not substantially favoured towards either of the two different metal
atom positions available in the Ta3ZrCy (y = 1–3) clusters. The energy difference
between the two isomers for these clusters varies between 0.024 and 0.222 eV (i.e.
below the expected accuracy of the DFT calculations). However, Zr atom substitution
is consistently favoured in these clusters towards the metal atom positions which
facilitate the C atoms always being bound to the Ta-Zr faces of the tetrahedral Ta3Zr
cluster, leaving the one Ta face unbound.


The Zr atom in all the Ta3ZrCy cluster isomers has a greater positive charge than the
equivalent Ta atom in the corresponding Ta4Cy cluster (see Appendix VII). Indeed the
Zr atom has a small positive charge in the bare Ta3Zr cluster, with the Ta atoms
possessing slight negative charges. This is consistent with the Zr atom having a lower
electronegativity than the Ta atom; 1.33 c.f. 1.50 (Pauling Scale). In the lowest energy
isomers of the Ta3ZrCy (y = 1–3) clusters the Zr atom is always bound to all the
electronegative C atoms, whereas the higher energy isomers do not have the Zr atom
bound to all the C atoms. The Zr atom possesses a greater positive charge in the
former isomers compared to the latter. Therefore, the slightly favoured Zr-C bonding
compared to Ta-C bonding is most likely due to the slightly greater ionic bonding
component of the Zr-C interaction being energetically favourable.



7.7.2 Electronic Structure of the Bimetallic-Carbide Clusters

Of interest is whether the Ta3ZrCy clusters have electronic structures similar to that of
the Ta4Cy clusters, or whether the differences between a Zr and Ta atom induce a
notable change.




                                            149
Chapter 7: Tantalum-Zirconium-Carbide Clusters




Figure 7-11: Illustrative representations for the HOMO and LUMO of the neutral
and cationic Ta4, Ta4C and Ta4C2 clusters and also for the corresponding neutral
Ta3Zr, Ta3ZrC and Ta3ZrC2 cluster isomers (see text for details).




                                      150
Chapter 7: Tantalum-Zirconium-Carbide Clusters


The neutral Ta3ZrCy clusters can be considered as being iso-electronic with the
cationic Ta4Cy clusters, yet not possessing an overall +1 charge, or as having one less
electron than the neutral Ta4Cy clusters, but possessing the same overall charge of
zero.


To investigate this question of electronic structure changes, the frontier orbitals (i.e.
the HOMO and LUMO) of the neutral Ta3ZrCy clusters and of the corresponding
neutral and cationic Ta4Cy clusters are examined to see if they are essentially identical
in arrangement or whether there are changes induced by Zr atom substitution. The
relevant HOMO and LUMO for the neutral and cationic Ta4Cy clusters and for the
neutral Ta3ZrCy clusters are shown in Figure 7-11 and Figure 7-12. Note that the
frontier orbitals for both isomers of the Ta3ZrCy (y = 1–3) clusters are examined.


In comparison to the Ta3ZrCy cluster isomers, it can be seen that the only iso-
electronic Ta4Cy+ cluster which has both corresponding frontier orbitals essentially
identical in arrangement is Ta4C+ (to the Ta3ZrCy A isomer). The remaining Ta4Cy+
clusters either have one frontier orbital essentially identical in arrangement to the
corresponding orbital in the Ta3ZrCy cluster isomers (i.e. Ta4C3+) or none (i.e. Ta4+,
Ta4C2+ and Ta4C4+).


All the Ta3ZrCy clusters and the majority of Ta4Cy clusters have doublet and singlet
multiplicity ground electronic states, respectively. Therefore, if frontier orbital
changes have not occurred upon substitution, the HOMO and LUMO of each cluster
will appear essentially identical in arrangement. In the cases of the Ta4C2 and Ta4C4
clusters, where the ground electronic states are of triplet multiplicity, the HOMO−1
and HOMO correspond to the HOMO and LUMO of the Ta3ZrC2 and Ta3ZrC4
clusters, respectively. However, for simplifying the current discussion, the HOMO
and LUMO of the singlet multiplicity states of Ta4C2 and Ta4C4 are used for
comparison.


The frontier orbitals of Ta3Zr are essentially identical in arrangement to those of Ta4.
Two isomers are possible for the Ta3ZrC, Ta3ZrC2 and Ta3ZrC3 clusters. The lowest
energy isomers for these clusters (i.e. A) have changes in their frontier orbitals



                                          151
Chapter 7: Tantalum-Zirconium-Carbide Clusters


compared to those of the corresponding Ta4Cy clusters. In these isomers for Ta3ZrC
and Ta3ZrC2 only one corresponding orbital is essentially identical in arrangement;
the LUMO for the former cluster and the HOMO for the latter cluster. In the lowest
energy isomer of Ta3ZrC3 neither the HOMO nor LUMO match those of Ta4C3. In
contrast, the low-lying isomers for all the Ta3ZrC, Ta3ZrC2 and Ta3ZrC3 clusters (i.e.
B) have both their frontier orbitals being essentially identical in arrangement to those
of the corresponding Ta3ZrC, Ta3ZrC2 and Ta3ZrC3 clusters, respectively. For
Ta3ZrC4 only one isomer is possible, with frontier orbital changes occurring compared
to the corresponding orbitals in Ta4C4.




Figure 7-12: Illustrative representations for the HOMO and LUMO of the neutral
and cationic Ta4C3 and Ta4C4 clusters and also for the corresponding neutral
Ta3ZrC3 and Ta3ZrC4 cluster isomers (see text for details).




                                          152
Chapter 7: Tantalum-Zirconium-Carbide Clusters


In summary, all of the neutral Ta3ZrCy cluster isomers (except Ta3ZrC A) display
changes in their frontier orbitals compared to those of the corresponding iso-electronic
cationic Ta4Cy clusters. Furthermore, the lowest energy isomers of the neutral Ta3ZrCy
clusters also have changes in their frontier orbitals compared to the corresponding
neutral Ta4Cy clusters. However, the low-lying isomers of the neutral Ta3ZrC,
Ta3ZrC2 and Ta3ZrC3 clusters do have frontier orbitals essentially identical in
arrangement to those of the corresponding neutral Ta4Cy clusters.


This difference between the two isomers appears to arise due to the higher oxidation
of the underlying Ta3Zr cluster in the lowest energy isomers, compared to the low-
lying isomers (see Appendix VII). Indeed the total charge on the metal atoms in all
the neutral Ta3ZrCy cluster isomers is equal to, or greater than, that in the
corresponding neutral Ta4Cy clusters. The difference between the total charge on the
metal atoms in a Ta3ZrCy cluster and that in the corresponding Ta4Cy cluster is always
greater in the lowest energy Ta3ZrCy isomer (0.14–0.28 electrons) compared to the
higher energy isomer (0.00–0.18 electrons). The difference in the total charge on the
underlying Ta3Zr cluster between the two isomers for each Ta3ZrCy (y = 1–3) cluster
varies from 0.10 and 0.20 electrons. As the greater charge on the metal atoms in the
Ta3ZrCy cluster isomers is not distributed in a uniform manner, this slightly greater
total charge on the underlying Ta3Zr cluster in the lowest energy isomers appears to
be great enough to cause changes in the frontier orbitals.



7.7.3 Comparison of IP Trends for the Ta3ZrCy and Ta4Cy Clusters

The experimental IPs for both the Ta3ZrCy and Ta4Cy (y = 0–4) clusters are plotted in
Figure 7-13, as a function of the number of C atoms attached. The IPs of Ta3Zr and
Ta3ZrC are slightly reduced from those of Ta4 and Ta4C (−0.10 and −0.14 eV
respectively). However, the IPs of Ta3ZrC2, Ta3ZrC3 and Ta3ZrC4 are all reduced,
compared to those of the corresponding Ta4Cy clusters, by similar intermediate values;
−0.49, −0.42 and −0.48 eV, respectively.




                                           153
Chapter 7: Tantalum-Zirconium-Carbide Clusters


                            6.0


     Experimental IP (eV)


                            5.5




                            5.0

                                      Ta C Expt. IP
                                        4   y



                                      Ta ZrC Expt. IP
                                        3       y




                            4.5
                                  0                     1            2           3   4
                                                            No. Carbon atoms (y)
Figure 7-13: Graph showing experimental IP values for the Ta4Cy (blue) and Ta3ZrCy
(red) cluster series as a function of y.

Overall the IP trend for the Ta3ZrCy cluster series is reasonably similar to that of the
Ta4Cy cluster series; i.e. clusters containing two and four C atoms have the greatest IP
reductions relative to the bare metal clusters, although the reductions are greater for
the former. Indeed, the IP of the Zr atom is lower than that of the Ta atom. However,
there is a distinct difference in the two IP trends for clusters containing three C atoms.
In the Ta4Cy cluster series the IP of Ta4C3 is very similar to that of Ta4 and Ta4C,
whereas in the Ta3ZrCy cluster series the IP of Ta3ZrC3 is appreciably reduced
compared to that of Ta3Zr and Ta3ZrC (see Figure 7-13). The Ta3ZrC3 cluster is the
only bimetallic-carbide cluster where the low-lying isomer (i.e. B) has a lower IP than
the lowest energy isomer (i.e. A); with the former in much better agreement with the
experimental IP and calculated to be only 0.132 eV higher in energy. Therefore, the IP
tends may be very similar between that of the Ta3ZrCy and Ta4Cy cluster series, if
only the calculated lowest energy isomers of the former were present (see Figure
7-10).


Note that as the threshold ionisation technique favours the detection of the isomer
present with the lowest IP, it is possible that the low-lying isomers of Ta3ZrC and
Ta3ZrC2 are also present in our experiment but their ionisation onsets are not



                                                                 154
Chapter 7: Tantalum-Zirconium-Carbide Clusters


discernable in the PIE spectra. This situation is quite likely as, like that for Ta3ZrC3,
the ∆E of the B isomers for these two clusters are quite small; +0.222 and +0.024 eV,
respectively.




                                          155
Chapter 7: Tantalum-Zirconium-Carbide Clusters



7.8 Summary

In summary, this chapter has shown that bimetallic-carbide clusters of the form
TaxZrCy are generated with our constructed double ablation source. However, this was
only possible by the procedure of ablating the Zr rod earlier than the Ta rod, despite
the latter being located downstream.


Also shown was that the isomeric structures of the Ta3ZrCy clusters can be inferred by
the determination of their IPs by PIE experiments in combination with DFT
calculations on candidate isomers. The good agreement between the Ta3ZrCy cluster
series experimental and calculated IP trends indicated that the energetically
favourable Ta3ZrCy isomers are generated by substitution of a Ta atom with a Zr atom
in the lowest energy isomers of the Ta4Cy clusters. For the Ta3ZrC, Ta3ZrC2 and
Ta3ZrC3 clusters, where two isomers are possible via substitution, the lowest energy
isomers were those where all the C atoms were always only bound to the metal faces
on the tetrahedral Ta3Zr cluster which contain the Zr atom. However, the low-lying
isomers of the Ta3ZrCy clusters where one of the C atoms is always bound to the only
metal face containing all Ta atoms on the tetrahedral Ta3Zr cluster, were only very
slightly higher in energy. The slightly favoured Zr-C bonding, compared to Ta-C
bonding, was reasoned to be due to the slightly greater ionic bonding component of
the former interaction being energetically favourable.


All the neutral Ta3ZrCy cluster isomers (except Ta3ZrC A) display changes in their
frontier orbitals compared to the corresponding iso-electronic cationic Ta4Cy clusters.
Overall the lowest energy isomers of the neutral Ta3ZrCy clusters have changes in
their frontier orbitals compared to the corresponding neutral Ta4Cy clusters. However,
the low-lying isomers of the neutral Ta3ZrC, Ta3ZrC2 and Ta3ZrC3 clusters do have
frontier orbitals essentially identical in arrangement to those of the corresponding
neutral Ta4Cy clusters.


The IP trend for the Ta3ZrCy cluster series was reasonably similar to that of the Ta4Cy
cluster series, although the IP reductions upon carbon addition were greater for the
former. The larger than expected IP reduction for the Ta3ZrC3 cluster was believed to


                                         156
Chapter 7: Tantalum-Zirconium-Carbide Clusters


be due to its low-lying isomer being the carrier of the ionisation onset, rather than the
lowest energy isomer which has a higher IP. Therefore, the IP tends between that of
the Ta3ZrCy and Ta4Cy cluster series may be very similar if only the calculated lowest
energy isomers of the former clusters are present.




                                          157
Chapter 7: Tantalum-Zirconium-Carbide Clusters



7.9 References

        (1)      Nonose, S.; Sone, Y.; Onodera, S.; Sudo, S.; Kaya, K. J. Phys. Chem.
1990, 94, 2744.
        (2)      Nakajima, A.; Kishi, T.; Sugioka, T.; Sone, Y.; Kaya, K. J. Phys.
Chem. 1991, 95, 6833.
        (3)      Koyasu, K.; Mitsui, M.; Nakajima, A.; Kaya, K. Chem. Phys. Lett.
2002, 358, 224.
        (4)      Koretsky, G. M.; Kerns, K. P.; Nieman, G. C.; Knickelbein, M. B.;
Riley, S. J. J. Phys. Chem. A 1999, 103, 1997.
        (5)      Knickelbein, M. B. Phys. Rev. B 2007, 75.
        (6)      Bouwen, W.; Thoen, P.; Vanhoutte, F.; Bouckaert, S.; Despa, F.;
Weidele, H.; Silverans, R. E.; Lievens, P. Rev. Sci. Instru. 2000, 71, 54.
        (7)      Neukermans, S.; Janssens, E.; Tanaka, H.; Silverans, R. E.; Lievens, P.
Phys. Rev. Lett. 2003, 90.
        (8)      Tanaka, H.; Neukermans, S.; Janssens, E.; Silverans, R. E.; Lievens, P.
J. Am. Chem. Soc. 2003, 125, 2862.
        (9)      Li, X.; Kiran, B.; Li, J.; Zhai, H. J.; Wang, L. S. Angew. Chem. Int. Ed.
2002, 41, 4786.
        (10) Zhai, H. J.; Li, J.; Wang, L. S. J. Chem. Phys. 2004, 121, 8369.
        (11) Cartier, S. F.; May, B. D.; Castleman Jr, A. W. J. Chem. Phys. 1994,
100, 5384.
        (12) Cartier, S. F.; May, B. D.; Castleman Jr, A. W. J. Am. Chem. Soc.
1994, 116, 5295.
        (13) Cartier, S. F.; May, B. D.; Castleman Jr, A. W. J. Chem. Phys. 1996,
104, 3423.
        (14) Sakurai, H.; Castleman Jr, A. W. J. Phys. Chem. A 1998, 102, 10486.
        (15) Munoz, J.; Pujol, C.; Bo, C.; Poblet, J.-M.; Rohmer, M.-M.; Benard,
M. J. Phys. Chem. A 1997, 101, 8345.
        (16) Asmis, K. R.; Santambrogio, G.; Brummer, M.; Sauer, J. Angew.
Chem. Int. Ed. 2005, 44, 3122.
        (17) Janssens, E.; Santambrogio, G.; Brummer, M.; Woste, L.; Lievens, P.;
Sauer, J.; Meijer, G.; Armis, K. R. Phys. Rev. Lett. 2006, 96, 233401.
        (18) Yang, D.-S.; Hackett, P. A. J. Electron Spectrosc. Relat. Phenom.
2000, 106, 153.
        (19) Wang, C.-C.; Zhoa, R.-N.; Han, J.-G. J. Chem. Phys. 2006, 124,
194301.




                                           158

								
To top