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Probing very long lived excited electronic states of molecular

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Probing very long lived excited electronic states of molecular Powered By Docstoc
					Probing very long-lived excited electronic states of
    molecular cations by mass spectrometry




                 Prof. Myung Soo Kim


                   School of chemistry and
           National Creative Research Initiative for
                Control of Reaction Dynamics,
                  Seoul National University,
                    Seoul 151-742, Korea
I. Introduction

 A. Excited electronic states

   Involved in various processes such as photochemistry,
    operation of lasers, etc.

   Difficult to probe. Information scarce.
     A frontier in physical chemistry research
      For example, accurate and efficient calculation of
      excited state energy is the main focus in quantum
      chemistry.

   Our interest  Utilization of excited electronic states
                   for reaction control
B. Fate of an isolated polyatomic system prepared
   in an excited electronic state

 1. Nonradiative decay
    Internal conversion / intersystem crossing convert the electronic
    energy into vibrational energy in the ground electronic state.

 2. Direct photodissociation on a repulsive state
    Utilized in our previous work on reaction control via
    conformation selection (Nature 415, 306 (2002)).

 3. Radiative decay – fluorescence
    Occurs when nonradiative decay is not efficient and
    electric dipole – allowed transition is present.
C. Excited electronic states of molecular ions


              LUMO


              HOMO




                       Hole states           LUMO states


      Electron ionization (EI) and VUV photoionization (PI) generate
       hole states mostly.
       Peaks in photoelectron spectrum  hole states.
      There are more excited electronic states near the ground state of
       a molecular ion than that of a neutral ( presence of hole states).
                                             
        Rapid internal conversion prevalent.
       Fluorescence hardly observed for polyatomic molecular cations.
D. Theory of mass spectra

 1. Quasi-equilibrium theory (QET)

  1) Molecular ions in various electronic (and vibrational) states
     are produced by EI (or PI).

  2) Ions in excited electronic states undergo rapid internal
     conversion to the ground state.  Rapid conversion of
     electronic energy to vibrational energy.

  3) Intramolecular vibrational redistribution (IVR) occurs rapidly
     also.  Transition state theory, or,
     Rice-Ramsperger-Kassel-Marcus (RRKM) theory.
      QET or RRKM – QET
                             Wi‡(E - E 0i )
                 ki (E)  i
                              hρ (E)
2. Test

  Prepare M+ with different E.
  Measure k i or product branching ratios vs. E.
  Compare with the calculated results.


3. Results

  RRKM-QET adequate for most of the cases studied.

  Some exceptions observed.
   : Mostly direct dissociation in repulsive excited states.
     In several cases, dissociation in excited states which do
     not undergo rapid internal conversion to the ground state
     suggested.
             ‘Isolated electronic state’
II. Initial discovery

 A. Photodissociation of benzene cation
                           Chopper
     Argon ion laser                                     Prism
                                     Laser beam
                                                          Lens

                                                                        Electric               J. Chem. Phys. 113, 9532 (2000)
                                                                        sector
                                     Ion beam
                                                 Electrode
                                                 assembly                           Phase-
                                                                                   sensitive   Schematic diagram of the double
                       Magnetic sector                                             detection   focusing mass spectrometer with
                                                 Laser beam
                                                                                               reversed geometry (VG ZAB-E) modified
                                         R1           R2R3R4R5R6R7
                                                                                               for photodissociation study. The inset
        Ion source                        Collision
                                            cell                                               shows the details of the electrode
                                                                                               assembly.
                                                                  Ion
                                                                 beam




    Observed C6H6+  C6H5+, C6H4+, C4H3+, C3H3+
     at 514.5nm (2.41eV), 488.0nm (2.54eV), 357nm (3.47eV)
    Instrument can detect PD occurring within ~1 sec.
        Electronic states             Dissociation
            ( C6H6+• )                ( Products )
E(eV)
  6
               ~2
               E B2u                  ktot ~ 107s-1
 5
               ~ 2
               D E1u                  ktot ~ 104s-1
                                                             Energy diagram of the benzene
 4                                                          molecular ion. The lowest reaction
                                      C6H6+•  C6H5+ + H•   threshold (E0) is 3.66 eV for
 3
               ~ 2                                          C6H6C6H5H. ktot denotes the
               C A2u
                                                            total dissociation rate constant in
               ~2
               B E2g                                        the ground state calculated from
 2                                                          previous results.

  1


               ~2
  0            X E1g (ground state)




   For PD to be observed with the present apparatus,
    photoexcited C6H6+ must have E > 5 eV
                                                                       Remainder ?
   Photon energy = 2.4 ~ 3.7 eV.
                                                    A




       Intensity
                          B




                   5300            5500          5700      5900
                              Translational energy, eV

   PD-MIKE profile for the production of C4H4 from the benzene ion at
357nm obtained with 2.1kV applied on the electrode assembly.
Experimental result is shown as filled circles. Reproduction of the profile
using the rate constant distribution centered at 6.3107 s-1 obtained by
experimental data is shown as the solid curve. The positions marked A and
B are the kinetic energies of products generated at the position of
photoexcitation and after exiting the ground electrode, respectively.
                  10
                                           357 nm
                                            PD

                  8
                                488.0 nm                     The total RRKM dissociation rate constant of
                                  PD
                                                            BZ as a function of the internal energy
 -1




                                                            calculated with molecular parameters in ref. 8.
  log k, k in s




                  6                                         The internal energies corresponding to the
                                                            dissociation rate constants of (5.51.1)107
                                                            and (53)106 s-1 for PDs at 357 and 488.0 nm,
                  4                                         respectively, are marked.


                  2


                       4   5                6       7   8

                               Internal energy, eV



 Excellent RRKM – QET fitting of k is known for C6H6+ dissociation.
 From measured                            k    E
   PD at 357nm (3.47eV)  E=6.1 ± 0.1eV  Initial E = 2.6 ± 0.1eV
   PD at 488nm (2.54eV)  E=5.5 ± 0.1eV  Initial E = 3.0 ± 0.1eV
Origin of internal E prior to photoexcitation

Most likely  vibrational energy acquired at the time of EI,
             either directly or via internal conversion from an
             excited electronic state.

2.6  0.1 eV for 357nm PD vs. 3.0  0.1 eV for 488nm PD ?
Experimental error?

Can we quench it by increasing benzene pressure in the ion
source, by resonant charge exchange ?

                C 6 H 6 +* + C 6 H 6  C 6 H 6 * + C 6 H 6 +
PD as a function of C6H6 pressure in the ion source
                                                              0.04 Torr
                                                                                               Pressure dependences of the precursor (BZ)
                                                                                              intensity (–––) and photoproduct (C4H4)
                                                                                              intensities at 357 (·····) and 488.0 (---) nm.
  Relative intensity




                                                 0.013 Torr                                   Pressure in the CI source was varied
                                                                                              continuously to obtain these data. Pressure was
                                                                                              read by an ionization gauge located below the
                                                                                              source. The inside source pressures estimated
                                                                          0.09 Torr           at three ionization gauge readings are marked.
                                                                                              The scale for the precursor intensity is different
                       0                                                                      from that for photoproduct intensities.
                                       -6               -5                      -4
                                     10              10                      10

                                  Ionization gauge reading, Torr




                       Pig/Torr     P/Torr     Zc/s-1           tR/s                Ncoll
                       410-6      0.0051        0.13             4.2                 0.6
                                                                                                      Ion source pressure (P), collision
                       110-5       0.013        0.33             5.8                 1.9
                                                                                                     frequency (Zc), source residence time (tR),
                       210-5       0.025        0.63             7.6                 4.8            and number of collisions (Ncoll) suffered by
                                                                                                     ions exiting the ion source at some
                       310-5       0.038        0.96             9.0                 8.6
                                                                                                     benzene pressures.
                       510-5       0.063        1.59            11.2                 18
                       710-5       0.088        2.23            13.0                 29
Quenching mechanism

 PD at 488nm efficiently quenched (by every collision)
   resonant charge exchange likely.


 PD at 357nm hardly quenched. Why?

  If C6H6+ undergoing PD at 357nm is in an excited
  electronic state,
           C6H6 +†+ C6H6  C6H6 + C6H6+†

  Population of C6H6 +† does not decrease by charge exchange.
Charge exchange ionization by benzene cation
in the ion source

 One of the ionization scheme classified as chemical
  ionization (CI), a useful ionization technique in mass
  spectrometry.
 Add small amount of sample (s) to reagent (R) 
  Electron ionization  Initially, R+ formed mostly.
 Charge exchange ionization of S by R+
  R+ + S R + S+, electron transfer
  Translational & vibrational energies are not important
  to drive this reaction
          E  IE (S)  IE (R)
  Occurs efficiently when E  0, exoergic reactions.
Relative intensity of S+ formed by charge exchange with C6H6+

  At low C6H6 pressure in the source  PD at 357nm occurs
    Possible presence of long-lived C6H6+, C6H6+†.

  At high C6H6 pressure  complete quenching of PD at 357nm
    absence of C6H6+†.
            Samples              IE (eV)   Low pressure   High pressure
      Chlorobenzene               9.06        3.6              3.5
         Fluorobenzene            9.20        3.9             1.4
          Benzonitrile            9.62        5.3             0.06
   Chloropentafluorobenzene       9.72        4.7             0.01
          Nitrobenzene            9.86        3.8             0.06
       Hexafluorobenzene          9.91        2.5             0.02        Ionization Energies and the ratios of
            Ethylene             10.51        3.0             0.02
                                                                          molecular ion intensities generated by
       Methylene chloride        11.32        4.4             0.04        charge exchange ionization (CI) with
           Chloroform            11.37        4.7             0.03        BZ and by electron ionization (EI).
       Carbon tetrachloride      11.47        3.4             0.06
             Ethane              11.52        0.09            ~0
     Dichlorofluoromethane       11.75        0.05            0.04
   1-chloro-1,1-difluoroethane   11.98        0.16            0.01
     Chlorodifluoromethane       12.20        0.09            0.05
            Methane              12.51        0.24            ~0
                          6   9.243 eV                    11.5 eV




                          4




            CI/EI ratio   2




                          0
                              9           10         11             12   13
                                         Ionization energy, eV




 C6H6+ generated at high P, fully quenched  ionizes samples
  with IE < 9.2eV.
  cf. IE (C6H6) = 9.243eV

 C6H6+ generated at low P  ionizes samples with IE < 11.5 eV.
                    ~
  cf. IE of C6H6 to A2 E2gstate of C6H6 = 11.488 eV
B. Summary

 Low-lying excited electronic states of C6H6+



            ~                   ~               ~
            X 2 E1g             A2 E 2g         B2 A2u
         IE = 9.243 eV       IE =11.488 eV   IE = 12.3 eV



     ~
   A2 E2g has a very long lifetime, ‘isolated state’.
    ~2    ~
        
   A E2g X2 E1g electric dipole – forbidden.
    Internal conversion must be inefficient also.
                         ~
   For states above A2 E2g ,internal conversion efficient.
    (Evidence – failure to ionize S with IE > 11.5 eV by charge
     exchange)
       C6H6 Photoelectron Spectrum



       ~
       X




             ~
             A




                             ~          ~
Sharp vibrational peaks for X2 E1g and A2 E2g .
III. Charge exchange ionization to detect M+†

                            J.Am. Soc. Mass Spectrom. 12, 1120 (2001).



 1. Energetics

    A+ + B  A + B+,     E ,    energy defect
    E  IE (B)  RE (A  )
      For A+ in the ground state,
    E  IE (B)  IE (A)

     E > 0, endoergic
        = 0, resonant
        < 0, exoergic
2. Charge exchange cross section

 1) Charge exchange between atomic species
    Massey’s adiabatic maximum rule
      Maximum cross section (max) occurs at the velocity
                          a E
                    v ~
                            h
      For E ~ 0 , max observed v ~ 0
      Otherwise, max observed at high v
2) Charge exchange involving molecular species

   Exoergic charge exchange (E < 0)
      Release of E as product vibration
      Energetically nearly resonant
      large  at near thermal velocity

   Endoergic charge exchange (E > 0)
      Small  at near thermal velocity. Usually keV impact
       energy needed.
      Reactant vibrational energy sometimes helps to
       increase , but not dramatically.

                           For near thermal collision
      Exoergicity rule
                            large when E  0
                            small when E > 0
3. Instrumentation
 1) Requirement
   Collision cell for conventional tandem mass spectrometry
                
          M  G m1 , m  , etc.,
                        2               fragment ions from M 
                         G

                                          
                                    M  , m1 , m  , etc
             
            M                             2




   Charge exchange
          M  G  M  G 
    For charge exchange at low impact energy, M+ must be
     decelerated.

    Should detect G+, which moves thermally inside the cell.
      Low yield.
       2) Instrumentation
                                                               Second collision cell

                                                                       EM

                                                        Ion
                                                        beam                           Electric sector

                                                                   Conversion
                                Magnetic sector
                                                                    dynode
First collision cell


                                                                         Conversion
                                                                          dynode
                                 First collision cell


                                       Collision
                                         Cell    Y-lens


                       Ion
                       Source

                                           Repeller
3) First Cell

      Ion source            Collision cell

                                                    
                                             M  , m1 , G    Magnetic
         M+
                                                              analyzer


         Vs                      Vc




  Type I ions ( formed by EI in the source) KI = eVs
  Type II ions ( formed by CID in the cell) KII = e [Vs+(m1/M)(Vs-Vc)]
  Type III ions ( formed from collision gas) KIII = eVc


  Magnetic analyzer : m/z = B2r2e2/2K
4) Second Cell

   Ion source               Magnetic analyzer     Collision cell   Electrostatic analyzer

                   M1 , M  , 
                    
                          2
                                            M1




        Vs                                              Vc




               
     Select M 1 by magnetic analyzer.
     Measure ion kinetic energy by electrostatic analyzer.
     Detect ions generated from collision gas
        
    (       KE of type III differs from those of Type I & II)
  4. Charge exchange data for C6H6+

            1) Second cell

                                       +                                  ~
                                     77 (MID)                  RE (C6H6+, A 2 E 2g ) = 11.488 eV
                                                               IE (CS2) =10.07 eV
                                              II
                                                               E = 10.07-11.488 = -1.418 eV
Intensity




                                                               Exoergic !
                                II                             Ion signal from collision gas
                                                               observed at eVc
                III                                II
                                                               Lifetime 20s or longer.
               3900          3930          3960         3990
                      Translational Energy, eV
2) First cell




               I   I
                               I       I
                                                            ~
                                               RE (C6H6+, A 2 E 2g ) = 11.488 eV
                                               IE (CS2) =10.07 eV
                                               IE (CH3Cl) = 11.28 eV

                                       I
                                               Exoergic !
                                               Ion signals from collision gas
                                               observed and can be identified.

                       I
         III               I       I       I
3) Relative yield of collision gas ions vs. impact energy
         ~
    When A2E2g state is fully quenched

     RE ( C6H6+, X 2 E1g ) = 9.243 eV
                                                        ~

                                         -1
                                        10
     Relative Yield, (A ) / (C6H6 )




                                                                                                                 IE, eV
     .
     +




                                                                                                  +
                                                                                1,3-C4H6
                                                                                   +
                                         -2                                     CS2
                                        10                                                    +
                                                                                CH3Cl
                                                                                          +
                                                                                                      1,3-C4H6    9.08
                                                                                CH3F
     .




                                         -3
     +




                                                                                      +
                                        10                                      CH4
                                                                                                      CS2        10.07
                                         -4
                                        10                                                            CH3Cl      11.28
                                         -5
                                        10                                                            CH3F       12.47
                                         -6
                                        10                                                            CH4        12.51
                                              0       200    400    600   800    1000
                                                  Primary Ion Translational Energy, eV
               ~
          When A2E2g state is present
                                                       ~
                             RE ( C6H6+, A2 E 2g ) = 11.488 eV
                                   -1
                              10
                                                                                                             IE, eV
Relative Yield, (A ) / (C6H6 )




                                                                                            +¡¤
.
+




                                                                          1,3-C4H6
                                                                             +
                                   -2                                     CS2
                              10
                                                                          CH3Cl
                                                                                        +
                                                                                                  1,3-C4H6    9.08
                                                                                    +
                                                                          CH3F
                                   -3
.




                                                                                +
                              10
+




                                                                          CH4
                                                                                                  CS2        10.07
                                   -4
                              10                                                                  CH3Cl      11.28
                                   -5
                              10                                                                  CH3F       12.47

                              10
                                   -6                                                             CH4        12.51
                                        0        200       400    600      800
                                            Primary Ion Translational Energy, eV
4. Summary

   Collision gas ion yield is dramatically enhanced when
    the charge exchange is exoergic.

   Detect charge exchange signal for various collision
    gases with different IE
     Presence / absence of a very long –lived state.
       Estimation of its RE.

    Or, charge exchange  energy titration technique to
    probe excited electronic states.
IV. Benzene derivatives                             J. Chem. Phys. In press, 2002.



 A. Halobenzenes

  C6H6       C6H5X        X         6b2 (Xnp∥ character)          2b1 (Xnp⊥ character)
                  3b1                                                          -
1e1g                                        -       +
                  1a2                                                      +

           6b2
           2b1
                              np




                                      ~
        e- removal from 3b1  (3b1)-1X 2 B
                                            1
                        1a2  (1a2) A A
                                   -1 ~ 2
                                                          Hole states appearing in
                                      ~ 2
                        6b2  (6b2)-1 B 2 B                photoelectron spectra
                                            2
                                   -1 ~ 2
                        2b1  (2b1) C B
                                                1
            C6H5Cl Photoelectron Spectrum


                      ~
                      B




             ~
             X




                                  ~
                                  2       2  ~
 Widths of vibrational bands of B B 2 & X B1 are comparable.
                                         ~
  Possibility of very long lifetime for B 2 B 2 of C6H5Cl+
            C6H5Br Photoelectron Spectrum



                    ~
                    B




               ~
               X




                                 ~2       2 ~
 Widths of vibrational bands of B B 2 & X B1 are comparable.
                                        ~
  Possibility of very long lifetime for B 2 B 2of C6H5Br+
     C6H5I Photoelectron Spectrum




  ~                          ~
 B 2 B 2 bands broader than X 2 B1
                          ~
   Rapid relaxation of B 2 B 2 of C6H5I+
    C6H5F Photoelectron Spectrum


           (F2p∥)-1




~
X




                                  ~2
   (F2p∥)-1   bands broader than X B1
     Rapid relaxation
B. Triple bonds

         C6H6   C6H5CN/ C6H5CCH    CX

                         3b1
      1e1g
                         1a2


                  6b2
                  2b1
                                       

    6b2  (CX∥) character
     2b1  (CX⊥) character

    e- removal from 3b1 
                                   ~
                                   X 2 B1
                    1a2           ~2       Hole states appearing in
                                   A A2
                    6b2           ~2        photoelectron spectra
                                   B B2
    ~
    B
~
X


                                          C6H5CCH
                                          Photoelectron Spectrum

~        ~
X        B




                                          C6H5CN
                                          Photoelectron Spectrum



                                 ~
    Sharp vibrational bands for B 2 B 2 states.
                                     ~
     Possibility of very long-lived B B 2 states of
                                      2


        C6H5CN+, C6H5CCH+.
C. Experimental results


 1) C6H5Cl+

            ~
   C6H5Cl+( B) + CH3Cl  C6H5Cl + CH3Cl+

                 ~
    RE (C6H5Cl+, B ) = 11.330 eV
    IE (CH3Cl) =11.28 eV
    E = 11.28 eV – 11.330 eV = -0.05 eV,   exoergic!



                               ~
   CH3Cl+ would be observed if B of C6H5Cl+ is very long-lived.
                           (a)                                                                                                         +




                                                                                                                                       I C4H3
                     100



                      50                                                                           .
                                                                                                   +                                                           .




                                                                                                   I C4H2
                                                                                                                                                               +




                                                                                                                                                                I C4H4
                       0
                           (b)                                                                                                         +                                  Partial mass spectrum of C6H5Cl generated by 20 eV EI



                                                                                                                                       I C4H3
                                                                                                                                                                         recorded under the single focusing condition with 4006 eV
Relative Intensity




                     100
                                                                                                                                                                         acceleration energy is shown in (a). (b) and (c) are mass
                                                      .                                                                                                                  spectra in the same range recorded with CH3Cl in the
                                                      +
                                                                                                   .                             .                                       collision cell floated at 3910 and 3960 V, respectively.
                                                      III CH3 Cl




                      50    +                                                                      +                             +
                                                                                                                                                             .
                                                             35




                                                                                                   I C4H2




                                                                                      +
                            III CH2 Cl




                                                                                                                         III CH337Cl
                                                                                      III CH2 Cl
                                   35




                                                                                                                                                             +
                                                                                                                                                                         Type II signals at m/z 49.3 and 50.3 in (b) and at m/z 49.6
                                                                                             37




                                                                                                                                                             I C4H4
                                                                                                            II




                                                                                                                                                                         and 50.6 in (c) are due to collision-induced dissociation of
                                                                                                                          II
                                                                   II




                       0
                                                                                                                                                                         C6H5Cl+ to C4H2+ and C4H3+, respectively. The peaks at
                           (c)                                                                                                         +
                                                                                                                                                                         m/z 50.6 in (b) and at m/z 50.8 in (c) are due to collision-
                                                                                                                                       I C4H3




                     100                                                                                                                                                 induced dissociation of C6H5+ to C4H3+.
                                                                        .
                                                                        +                                        +
                                                                        III CH335Cl




                                                                                                                 II/III CH2 Cl




                                                                                                                                                .
                                                                                                                           37




                                                                                                                                                +
                      50                                                                           .                                                         .
                                                                                                                                                III CH3 Cl




                                         +
                                                                                                                                                37
                                         III CH2 Cl




                                                                                                   +                                                         +
                                                                                                   C4H2
                                                35




                                                                                                                                                             I C4H4
                                                                                                   I



                                                                                                                                 II
                                                                                      II




                       0
                                 48                          49                                50                                    51                      52
                                                                                      m/z
2) C6H5Br+

         ~
 C6H5Br+(B ) + CH3Br  C6H5Br + CH3Br+

                ~
   RE (C6H5Br+, B ) = 10.633 eV
   IE (CH3Br) =10.54 eV
   E = 10.54 eV - 10.633 eV = -0.093 eV,   exoergic!


                              ~
  CH3Br+ would be observed if B of C6H5Br+ is very long-lived.
                                                    .
                                                    +                          .
                                                                               +




                                                    III CH3 Br




                                                                               III CH381Br
                                                           79
Relative Intensity
                     100
                                      +
                                                                  +




                                      III CH279Br




                                                                  III CH2 Br
                                                                         81
                     50



                      0

                           88    90                         92                           94   96
                                                                 m/z

               Partial mass spectrum obtained under the single focusing condition
              with C6H5Br and CH3Br introduced into the ion source and collision cell,
              respectively. C6H5Br was ionized by 20 eV EI and acceleration energy
              was 4008 eV. Collision cell was floated at 3907 V.
3) C6H5CN+

        ~
C6H5CN+(B ) + CH3Cl  C6H5CN + CH3Cl+

                 ~
   RE (C6H5CN+, B ) = 11.84 eV
   IE (CH3Cl) =11.28 eV
   E = 11.28 eV – 11.84 eV = -0.56 eV,   exoergic!


                              ~
 CH3Cl + would be observed if B of C6H5CN+ is very long-lived.
                                                  .
                                                  +




                                                  III CH335Cl
                                                                                                                              .
                                                                                    .                                         +




    Relative Intensity




                                                                                                                              I C4H4
                                                                                    +
                         100




                                                                                        I C4H2
                                    +                                                                      .




                                    III CH235Cl
                                                                                                           +




                                                                                                           III CH3 Cl
                                                                                                                  37
                                                                          +
                         50




                                                                          III CH237Cl

                                                                                                 II
                                                                II




                                                                                                                         II
                                                                     II




                                                                                                      II
                          0

                               47         48             49                      50                                 51        52
                                                                     m/z

  Partial mass spectrum obtained under the single focusing condition with C6H5CN and
CH3Cl introduced into the ion source and collision cell, respectively. C6H5CN was
ionized by 20 eV EI and acceleration energy was 4007 eV. Collision cell was floated at
3910 V. Type II signals at m/z 49.3, 50.3, and 51.3 are due to collision-induced
dissociation of C6H5CN+ to C4H2+, C4H3+, and C4H4+, respectively. Those at m/z 49.6
and 50.6 are due to collision-induced dissociation of C6H4+ to C4H2+ and C4H3+,
respectively.
4) C6H5CCH+

          ~
 C6H5CCH+(B ) + CS2  C6H5CCH + CS2+

                   ~
   RE (C6H5CCH+, B ) = 10.36 eV
   IE (CS2) =10.07 eV
   E = 10.07 eV - 10.36 eV = -0.29 eV,   exoergic!


                           ~
 CS2+ would be observed if B of C6H5CCH+ is very long-lived.
                                               .
                                               +




                                               III C32S2
Relative Intensity
                     100



                     50
                                                                . .
                                                                  +




                                                                         III C S S
                                                                +




                                                                              32 34
                                                                I C6H4
                                                           II
                                     II
                      0
                           70   72        74                76                        78
                                           m/z

  Partial mass spectrum obtained under the single focusing condition with
C6H5CCH and CS2 introduced into the ion source and collision cell, respectively.
C6H5CCH was ionized by 14 eV EI and acceleration energy was 4006 eV.
Collision cell was floated at 3942 V. Type II signals at m/z 73.5 and 75.7 are due
to collision-induced dissociation of C6H5CCH+ to C6H2+ and C6H4+,
respectively.
Collision gases, their ionization energies(IE) in eV , and success / failure
to generate their ions by charge exchange with some precursor ions


                                                Precursor ion
   Collision gas   IE, eV
                            C6H5Cl+• C6H5Br+•   C6H5CN+•   C6H5CCH+• C6H5I+•   C6H5F+•

    (CH3)2CHNH2     8.72      O         O                       O        O
    1,3-C4H6        9.07                            O                    X      O
    (butadiene)
    CS2            10.07                                         O
    CH3Br          10.54      O        O            O            X      X
    C2H5Cl         10.98               X
    CH3Cl          11.28      O        X            O                           X
    C2H6           11.52      X                     O
    O2             12.07                            X
    Xe             12.12      X                    X                            X
    CHF3           13.86                                                        X
                         ~
   Recombination energy (X)  9.066     8.991       9.71         8.75   8.754   9.20
                         ~
   Recombination energy (B) 11.330   10.633       11.84     10.36      9.771   13.81*
                                  ~      ~         ~
Recombination energies of the X2B1, A2A2, and B2B2 states and the
                                                           ~
oscillator strengths of the radiative transitions from the B2B2 states.

    State          C6H5Cl+      C6H5Br+         C6H5I+      C6H5CN+   C6H5CCH+
    ~2                9.066        8.991           8.754          9.71        8.75
    X B1
                  (0.0000000)   (0.0000000)     (0.0000000)   (0.0000000) (0.0000000)

    ~2               9.707         9.663           9.505         10.17        9.34
    A A2          (0.0000008)   (0.0000001)     (0.0000000)   (0.0000010) (0.0000004)
    ~2
    B B2            11.330        10.633          9.771         11.84       10.36

 Lowest quartet     13.236        13.381         12.664         13.3         12.7

Reaction threshold 12.356         11.891         11.07          12.725       12.41


                      ~
 Radiative decay of B2B2 is not efficient for all the cases.
  ~
 B states are not dissociative.
                                                  ~
 The lowest quartet states lie ~2 eV above the B state. Relaxation by
  doublet – quartet intersystem crossing would not occur.
 Internal conversion must be inefficient for the ~ states except for
                                                  B
                  ~ state of C H I+, internal conversion must be efficient.
  C H I+. For the B
    6   5                              6    5
V. Vinyl derivatives

 A. Detection of Type III ions by double focusing
    mass spectrometry
             Type I : KI = eVS
             Type II : KII = e[VC + (m2/m1)(VS - VC)]
             Type III : KIII = eVC
                                       Magnetic analyzer Electrostatic analyzer
     Ion source       Collision cell



        Vs                  Vc


     Scheme
      1. Set the electrostatic analyzer (kinetic energy analyzer) to
         transmit ions with kinetic energy eVc.
      2. Scan the magnetic analyzer (momentum analyzer, or
         mass analyzer).

                     Detect Type III ions only.
      B. Vinyl halide

    C2H4        C2H3X      X             a ( Xnp∥ character)    a ( Xnp⊥ character)


                   a

               a
                               Xnp
               a




                                         ~
            e- removal from a (C=C)  X 2 A
                                          ~
                             a (Xnp∥)  A 2 A        Hole states appearing in
                                                        photoelectron spectra
                                           ~
                             a (Xnp⊥)  B 2 B
1) Vinyl chloride




                                         ~2
            Sharp vibrational bands for A A 
               Possibility of very long lifetime.
2) Vinyl bromide




                                         ~
            Sharp vibrational bands for A 2 A 
              Possibility of very long lifetime.
3) Vinyl iodide

              ~
              A


          ~
          X




                                            ~
               Sharp vibrational bands for A 2 A 
                   Possibility of very long lifetime.
C. CH2=CHCN, Acrylonitrile




            ~
            X      ~
                   A




                                                 ~
           Possibility of very long lifetime for A 2 A 
D. CH2=CHF, Vinyl fluoride




              ~
              X
                     ~
                     A




                      ~
               Broad A 2 A  bands
                                       ~
                   Short lifetime for A 2 A 
E. Experimental results


 1) CH2=CHCl+

             ~
   CH2=CHCl+(A) + CH3Cl  CH2=CHCl + CH3Cl+

                       ~
     RE (CH2=CHCl+, A ) = 11.664 eV
     IE (CH3Cl) =11.28 eV
     E = 11.28 eV – 11.664 eV = -0.384 eV,   exoergic!


                               ~
   CH3Cl+ would be observed if A of CH2=CHCl+ is very long-lived.
  Relative Intensity    (a)                                             I   I

                                                                                     Single – focusing mass spectrum
                                  I                                                  recorded for C2H3Cl with CH3Cl
                                                                                     introduced to the first cell.
                              I                       III

                       20             30   40         50            60          70


                                                      III
Relative Intensity




                        (b)                                    35
                                                            CH3 Cl
                                                                    +




                                                  III III                            Double – focusing mass spectrum
                                                        III

                       20             30   40         50            60          70
                                                m/z

                              ~
                              A state of CH2=CHCl+ is very long-lived.
2) CH2=CHBr+

           ~
 CH2=CHBr+(A) + CH3Br  CH2=CHBr + CH3Br+

                     ~
   RE (CH2=CHBr+, A ) = 10.899 eV
   IE (CH3Br) =10.54 eV
   E = 10.54 eV – 10.899 eV = -0.359 eV,   exoergic!


                              ~
  CH3Br+ would be observed if A of CH2=CHBr+ is very long-lived.
                          (a)                  III                 III

Relative Intenxity


                                         III                 III
                                I   I                                            III

                     90             92         94                  96                  98

                      (b)                      III                 III
Relative Intensity




                                                        79     +            81     +
                                                     CH3 Br              CH3 Br




                                         III                 III
                                                                                 III

                     90             92         94                  96                  98
                                               m/z


                           ~
                           A state of CH2=CHBr+ is very long-lived.
3) CH2=CHI+

           ~
  CH2=CHI+(A ) + CH2=C=CH2  CH2=CHI + CH2=C=CH2+

                   ~
   RE (CH2=CHI+, A ) = 10.08 eV
   IE (allene : CH2=C=CH2) =9.69 eV
   E = 9.69 eV – 10.08 eV = -0.39 eV,   exoergic!



                                  ~
 CH2=C=CH2 + would be observed if A of CH2=CHI+ is very long-lived.
                                                III
                        (a)



Relative Intensity


                                          III                III



                       35         37      39                 41    43

                                                III
                            (b)                          +
                                                      C3H4
  Relative Intensity




                                          III                III


                       35         37      39                 41    43
                                         m/z


                        ~
                        A state of CH2=CHI+ is very long-lived.
4) CH2=CHCN+

             ~
   CH2=CHCN+(A) + Xe  CH2=CHCN + Xe+

                     ~
   RE (CH2=CHCN+, A ) = 12.36 eV
   IE (Xe) =12.12 eV
   E = 12.12 eV – 12.36 eV = -0.24 eV,   exoergic!



                           ~
  Xe+ would be observed if A of CH2=CHCN+ is very long-lived.
                                            III                III
                         (a)
                                                        III


Relative Intensity
                                                                     III   III

                                                  III
                                     III


                       124     126   128          130         132    134   136   138   140

                                            III               III
                         (b)
                                                        III
  Relative Intensity




                                                                     III
                                                                           III

                                                  III
                                      III


                       124     126   128          130         132    134   136   138   140
                                                              m/z


                        ~
                        A state of CH2=CHCN+ is very long-lived.
5) CH2=CHF+

            ~
  CH2=CHF+( A) + CH3F CH2=CHF + CH3F+


                    ~
   RE (CH2=CHF+, A ) = 13.80 eV
   IE (CH3F) =12.50 eV
   E = 12.50 eV – 13.80 eV = -1.3 eV,   exoergic!


                             ~
  CH3F+ would be observed if A of CH2=CHF+ is very long-lived.
                       (a)                   I




 Relative Intensity
                                                           Mass spectrum of C2H3F generated
                                                          by 20 eV EI recorded under the single
                                                          focusing condition without CH3F.




                      20     30         40           50

                       (b)                   I
Relative Intensity




                                                           Mass spectrum of C2H3F generated
                                                          by 20 eV EI recorded under the single
                                                          focusing condition with CH3F.
                                                 I

                                             I

                      20     30         40           50
                                  m/z




           ~
           A state of CH2=CHF+ is not long-lived.
                                            Precursor ions   Collision
gas IE, eV
                        C2H3Cl+• C2H3Br+• C2H3I+• C2H3CN+• C2H3F+•

1,3-C4H6
               9.07       O         O        O                  O
(butadiene)
C 3 H4
               9.692                         O
(Allene)
CH3Br         10.54       O         O        X         O

CH3Cl         11.28       O         X                           X

Xe            12.12       X                            O

CH3F          12.50                                    X        X

Ar            15.76                                             X
                      ~
Recombination energy (X) 10.005   9.804    9.35    10.91     10.63
                      ~
Recombination energy (A) 11.664   10.899   10.08   12.36     13.80
VI. Conclusion

 1. Charge exchange ionization has been developed as a
    useful technique to find very long-lived excited electronic
    states of polyatomic ions and estimate their recombination
    energies.
 2. The following very long-lived excited electronic states
    have been found.
                 +,     ~2                          ~
                                               +, A 2 A 
          C 6H 6        A E 2g       CH2CHCl
                        ~2                          ~
          C6H5Cl    +,  B B2         CH2CHBr+, A 2 A 
                        ~2                          ~
          C6H5Br    +,
                        B B2         CH2CHI+, A 2 A 
                     +,
                        ~2                          ~
                                                 +, A 2 A 
          C6H5CN        B B2         CH2CHCN
                            ~2
           C 6H 5   CCH+,   B B2
     Much more than found over the past 50 years!

				
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