1. INFRARED-ACTIVE VIBRON BANDS ASSOCIATED WITH RARE GAS SUBSTITUTIONAL IMPURITIES IN SOLID HYDROGEN PAUL L. RASTON and DAVID T. ANDERSON, Department of Chemistry, University of Wyoming, Laramie, WY 82071 NeH2H2 ArKrXe

2. * Important for understanding basic van der Waals interactions. * Model systems with regard to determination of accurate potential energy surfaces from experimental data. * Attractive to study because of the relative simplicity and fundamental nature of these systems. H 2 - Rg: R. J. Le Roy and J. M. Hutson, J. Chem. Phys. 86, 837 (1987). H 2 -H 2 : P. Diep, and J. K. Johnson, J. Chem. Phys. 112, 4465 (2000). H 2 - Rare gas (Rg) Systems

3. E. J. Allin, W. F. J. Hare, and R. E. MacDonald, Phys. Rev. 98, 554 (1955). * Isolated gas phase nH 2 infrared inactive. * High pressure gas phase  intermolecular interactions allow for infrared activity. * Low temperature condensed phase  longer duration of molecular interactions give rise to sharper spectra. * The induced Q 1 (0) (vibrational) and S 1 (0) (vibrational - rotational) transitions are reported for Rg doped solid pH 2 in this study. Gas Phase (high pressure) vs. Condensed Phase Spectra of nH 2 Q 1 (0) S 1 (0)

4. room temperature dopant Rg vacuum shroud radiation shield optical substrate T = 2.4 K IR beam atmosphere vacuum Fe(OH) 3 nH 2 cold tip of cryostat T = 14.5 K Rapid Vapour Deposition pH 2 S. Tam, and M. E. Fajardo, Rev. Sci. Instrumen. 70, 1926 (1999).

5. Pure pH 2 - net  = 0Doped pH 2 - net  ≠ 0 * In pure pH 2 Q 1 (0) transition will not be observed * The presence of an impurity Rg atom will induce a net dipole moment  observed transition. pH 2 Rg v=1 pH 2 Impurity Induced Transitions

6. v=0, j=0 v=1, j=0 * Q 1 (0) Transition (4153 cm -1 ) not allowed in pure pH 2. Dopant Induced pH 2 Q 1 (0) Vibron Band

7. * Larger rare gas atoms induce a greater transition moment in the surrounding pH 2 molecules  There is an exponential increase in the intensity of the induced Q 1 (0) feature in going from Ne to Ar to Kr to Xe. Trends in Q 1 (0) Rg Induced Region * Data also fits the prediction based on the relative potential well depths, i.e. the stronger the Rg-H 2 interaction, the further red-shifted is the H 2 ’s vibrational frequency.

8. R. J. Hinde, J. Chem. Phys. 119, 6 (2003). ΔE=0.25cm -1 ΔE=2.0cm -1 Theoretical models of impurity induced infrared-active vibron bands Calculated impurity-induced Q 1 (0) spectrum as a function of the detuning parameter ΔE (ΔE: magnitude the dopant shifts the Q 1 (0) vibrational frequency from its value in pH 2 ) * ΔE=2.0cm -1 induced vibron band comparable to Xe induced Q 1 (0) band * ΔE=1.5cm -1 induced vibron band comparable to Kr induced Q 1 (0) band * ΔE=0.25cm -1 induced vibron band comparable to Ar induced Q 1 (0) band

9. Delocalized vibron Localized vs. Delocalized Vibron Localized vibron

10. v=0, j=0 v=1, j=2 * S 1 (0) transition (4486 cm -1 ) allowed even in pure pH 2. S 1 (0) Rg Induced Transitions in Solid pH 2

11. * Complicated spectrum – why a minimum of 6 satellite peaks… S 1 (0) Xe Induced Transitions in Solid pH 2

12. v=1, j=2 pH 2 Computed Anisotropic Potential for H 2 in Lattice Xe H 2 - Xe: R. J. Le Roy and J. M. Hutson, J. Chem. Phys. 86, 837 (1987). H 2 -H 2 : P. Diep, and J. K. Johnson, J. Chem. Phys. 112, 4465 (2000).

13. * Inherent axial symmetry to each substitutional crystal site in hcp lattice  lifting of the J=2 upper state giving three rotational states with m J = ±2, ±1, and 0. * Two different substitutional sites exist for a Xenon impurity in a pH 2 hcp lattice, in-plane (IP) and out-of-plane (OP)  Potential explanation for the 3 x 2 = 6 excited state levels… 356.02 cm -1 354.54 cm -1 355.28 cm -1 357.56 cm -1 Calculated magnitude of splitting of J=2 level by Xe in hcp lattice 3cm -1 (total observed splitting is ~1.5cm -1 ) m J =±1 m J =±2 mJ=0mJ=0

14. IPOP * Calculations predict the difference between IP and OP substitution sites is not significant enough to cause further lifting to the degree which is observed experimentally. * Likely scenario is that Xe distorts the local lattice resulting in 2 more distinctly different pH 2 environments than considered in the calculations. In-Plane vs. Out-of-Plane Substitution

15. 356.02 cm -1 354.84 cm -1 355.43 cm -1 357.24 cm -1 2.4cm -1 m J =±1 m J =±2 mJ=0mJ=0 Calculated Potential Surface and J=2 Splitting for Kr Dopant Kr H 2 - Kr: H. Wei, and R. J. Le Roy, J. Chem. Phys. 122, 84321 (2005). H 2 -H 2 : P. Diep, and J. K. Johnson, J. Chem. Phys. 112, 4465 (2000). * Total calculated splitting induced by Kr is less than that for Xe (3cm -1 ). * This is to be expected as Kr-H 2 interaction is weaker.

16. Conclusions * The line shapes of the Q 1 (0) features induced by Rg atoms in solid pH 2 provide information on the extent of localization of the vibron. * The intensity of the Rg atom induced Q 1 (0) feature provides information on the induction mechanism and on the Rg-pH 2 intermolecular potential. * Satellite lines in the S 1 (0) region induced by the presence of Rg atoms provide information on the anisotropy of the Rg-pH 2 potential, and on the anisotropy of the crystal structure when the pH 2 molecule is in a J=2 rotational state. Future Work * Calculations that take into account distortions in the hcp lattice induced by Xenon…

17. Acknowledgements * M. E. Fajardo * R. J. Hinde * R. J. Le Roy * B. D. Lorenz * G. V. Subrahmanyam * Funding Sources: *Petroleum Research Fund *The Research Corporation *National Science Foundation

18. End

21. IsotopeAtomic mass (amu)Natural abundance (%) 124 Xe123.90589420.09 126 Xe125.9042810.09 128 Xe127.90353121.92 129 Xe128.904780126.44 130 Xe129.90350944.08 131 Xe130.90507221.18 132 Xe131.90414426.89 134 Xe133.90539510.44 136 Xe135.9072148.87

22. ComplexVo (cm-1)Ro (*10-10 m)Re (*10-10 m) D2-D22993.963.6423.478 H2-H24162.063.7893.478 H2-He D2-He H2-Ne4161.153.99 3.3 D2-Ne2993.523.66 H2-Ar4160.083.94 3.59 D2-Ar H2-Kr4159.544.07 3.72 D2-Kr H2-Xe4158.664.25 3.94 D2-Xe * All values are gas phase experimental or theoretical.