Helium Droplets Study of Hydrogen Bonding in Water Clusters by HC121105075854

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									CARS Spectra of Cold Hydrogen Clusters




                Kirill Kuyanov


             Department of Chemistry,
          University of Southern California



      OSU Molecular Spectroscopy Symposium
            Columbus, June 21, 2007
 Quantum properties of molecular hydrogen



                    He                            H2                            Ar
Boson               Yes                  +        Yes                  +        Yes          +

Λ*                  2.87                 +        1.97                 +        0.20         -

Interaction         11 cm-1              +        34.8 cm-1            -        143 cm-1     -


De Boer parameter: Λ* = Λ/R0
where Λ – de Broglie wavelength, R0 – distance of closest approach

        Small (n ≤ 30) para-hydrogen clusters are
        predicted to stay liquid down to 0 K and are
        proposed to become superfluid at about 2 K1)
        1)   P. Sindzingre, D. Ceperley, and M. L. Klein, Phys. Rev Lett. 67, 1871 (1991).
                  CARS studies of molecular H2



                                                 CARS spectroscopy –
                                                 tool to study hydrogen
                                                 clusters
                  H2 molecule

    I=0                           I=1
para-hydrogen                   ortho-hydrogen
 J = 0, 2, 4, …             J = 1, 3, 5, …
 levels allowed             levels allowed                           ν=1
                                                                     ν=0
CARS on H2 clusters: experimental setup                      H2




                                                             He




Dye laser    Nd:YAG




                       Level-up
                                                  T = 17 K


                                  P = 10-7 mbar


  PMT
                          CARS spectrum of clusters of para-H2 molecules in
                                            He droplets
                                                          Cluster core

                      5




                                                                                            Free p-H2 Molecules
                      4
Intensity, arb. un.




                      3                    p-H2 in bulk                  Cluster surface
                      2
                                 X 15
                      1

                      0

                          4135      4140      4145         4150          4155        4160                     4165
                                                                           -1
                                                     Wavenumber, cm
        Frequencies of Raman transitions in p-H2 molecules
              in cluster as a function of cluster size

Bulk solid p-H2
transition frequency 4149.5
               -1
                Raman frequency, cm




                                      4150.0

                                                                                                    p-H2
                                                                                                    clusters
                                      4150.5



                                      4151.0



                                      4151.5

   Molecular p-H2
                                               0.01       0.1          1          10          100
   transition frequency
                                                      Percentage of p-H2 in helium in expansion
   at 4161.1 cm-1
                         Phase separation in quantum liquids

                                4He and 3He are separated in droplets
                                due to difference in zero point energy:

                                4He    goes to the center of 3He droplet




Toennies, Vilesov, Whaley, Physics Today, Feb (2001) 35
Mixed ortho-/para- H2 clusters in He droplets
                                                            ortho-H2 concentration:
                      4
                                                                             75%
                      2
                      0
                      4                                                      63%
                      0
                                                                             50 %
                      1
Intensity, arb. un.




                      0
                      4
                                                                             40 %
                      2
                      0
                      4                                                      30 %
                      2
                      0
                                                                             20%
                      2
                      0
                      4                                                      10%
                      2
                      0
                      4                                                       0%
                      2
                      0
                          4135   4140   4145     4150      4155      4160       4165
                                                              -1
                                               Wavenumbers, cm
Frequencies of Raman transitions in hydrogen in
         bulk samples and in clusters

                       4154
                                  para-H2
 -1




                       4152
 Raman frequency, cm




                       4150
                                                             Bulk H2
                                                             H2 Clusters in
                       4148                                  He droplets
                                                             Neat H2 Clusters
                       4146

                       4144
                                   ortho-H2
                       4142
                              0   10    20    30   40   50   60     70    80
                                       ortho-H2 concentration, %
                           Enhancement of ortho-H2 Raman cross section in
                                        o-H2/p-H2 mixtures



                                                                                             Ortho-H2 molecules have
o-H2 Raman cross section




                                                                                             greater transition probabilities
   enhancement factor




                                                                                             when para-H2 molecules are
                                                                                             among their neighbors




                              ortho-H2 part in o-H2 /p-H2 crystal


                             J. van Kranendonk, Solid hydrogen. Theory of the properties of solid H2, HD and D2. 1983, Plenum Press
                                  Frequencies of Raman transitions.
                                 Corrected by the enhancement factor.

                      4154
                                                                              Para-H2 molecules’
                                                                              transitions in clusters
                      4152
-1




                                                                              are blue-shifted from
Raman frequency, cm




                      4150
                                                                              values in bulk.
                                                           Bulk H2
                      4148                                 H2 clusters in     Ortho-H2 molecules’
                                                           He droplets        transitions are at about
                      4146                                                    the same frequencies

                      4144

                      4142
                             0   10    20   30   40   50   60     70     80
                                      ortho-H2 concentration, %
Phase separation effect in mixed o-H2/p-H2 clusters

       Frequency shifts of H2 cluster lines and their
       relative intensities suggest quantum effect of
       phase separation in solid p-H2/o-H2 clusters:


                           He              ortho-H2 core




                                           para-H2 shell
                        Summary


•Clusters of molecular hydrogen embedded in helium droplets
were studied by CARS spectroscopy

•para-H2 clusters of sizes N ≈ 103 – 106 were investigated. As
cluster size increases, frequency of Raman transitions
approaches the bulk solid limit.

•Mixed ortho-H2/para-H2 clusters were studied Intensities and
frequency shifts in CARS spectra of analyzed

•Quantum phase separation effect was suggested in mixed orto-
H2/para-H2 clusters
Acknowledgments

Bobby Khalajestani
Russell Sliter
Mikhail Slipchenko
Boris Sartakov
Dmitry Skvotrsov


Andrey Vilesov


  N$F
     CARS spectra of large H2 clusters (n ~ 104) in
helium droplets (0.5% H2 in He: 40% o-H2; 60% p-H2)

                      Assigned to:          ortho-H2 in para-H2 in
                                              clusters   clusters



                                            o-H2 in bulk




                                                                  p-H2 in bulk




                                                                                  molec. o-H2
                      2.0




                                                                                                   molec. p-H2
Intensity, arb. un.




                      1.5

                      1.0                                                        Q1(1)

                      0.5                                                                        Q1(0)

                      0.0

                            4135     4140                  4145          4150    4155           4160             4165
                                                                                     -1
                                                              Wavenumber, cm
   CARS spectrum of p-H2 molecules in He droplets


                                                                                                 -1
                      2.0   13% p-H2 in He                                              4161.12 cm
Intensity, arb. un.




                                                                                         -1
                      1.5                                                  = 0.085 cm




                      1.0
                                                                                   -1
                                                              =11.4 cm
                                                -1
                      0.5             = 0.21 cm
                                                                     -1
                                                            4149.77 cm



                      0.0
                            4140    4145             4150        4155              4160               4165
                                                                          -1
                                                Wavenumber, cm
                                                 Concentration of ortho-H2:
                                           in clusters vs in pre-expansion mixture

Concentration of ortho-H2 in clusters, %   100


                                            80


                                            60


                                            40


                                            20


                                             0
                                                 0      20       40       60       80       100

                                                 Concentration of ortho-H2 before expansion, %
Neat H2 clusters: different ortho-H2/para-H2 ratios
                                                                   Concentration of o-H2
                        4                                                           75%
                        2
                        0

                        6                                                          60%
                        4
                        2
                        0
  Intensity, arb. un.




                        6                                                          50%
                        4
                        2
                        0
                        6
                        4                                                         25%
                        2
                        0

                        4                                                         10%
                        2
                        0
                        6
                        4                                                          0%
                        2
                        0

                            4135   4140   4145       4150     4155         4160      4165
                                                              -1
                                                 Wavenumber, cm
   Fundamental band of CO2 in He droplets at high laser flux


                                        0.4

                                        0.3
                        depletion (%)
                                        0.2                  SB = 15 cm-1
                                        0.1                                           5
                                        0.0                  SB

                                        2.0
                                                     R(0)     I(SB)/I(R(0)) = 0.04
                                        1.5

                                        1.0

                                        0.5

                                        0.0
                                              2300     2350        2400        2450    2500
                                                                          -1
                                                            wavenumber (cm )
Hoshina, Lukrezi, Slipchenko, Kuyanov, Vilesov,
           PRL 94, 195301 (2005).
                     Water molecule in helium droplets
                                                                                                                      220                  o

                                                                                                                      202                  o
                                                                                                  33   cm-            110                  p
• Helium droplets – superfluid                                                                    1
                                                                                                                      111                  o
                                                                                                                      101                  p
 matrix for molecular spectroscopy                                                                                    000                  o
• He droplet temperature is 0.37 K
• Free rotation and vibration of                                                                                            ν3 (a-type)

 molecules in helium droplets                                                                   Δ = 6 cm-1
                                                                                                                      101                  o
• Narrow spectral lines of molecules                                                                                  000                  p
                              Intensity, arb. un.




            He                                       0.4

                                                                                                            -1
                                                                                               = 0.4 cm                             -1
                                                                                                                         = 2.3 cm



                                                     0.0

                                                            3720           3740           3760               3780           3800
                                                                                                                 -1
                                                                                     Wavenumber, cm
                                                    K.E. Kuyanov, M.N. Slipchenko, and A.F. Vilesov. Chem Phys Lett, 2006. 427 (1-3): p. 5-9

								
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