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 NeH2H2 ArKrXe

* 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

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)

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).

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

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

* 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.

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

Delocalized vibron Localized vs. Delocalized Vibron Localized vibron

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

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

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).

* 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… cm cm cm 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

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

cm cm cm cm cm -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, (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.

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…

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

End

IsotopeAtomic mass (amu)Natural abundance (%) 124 Xe Xe Xe Xe Xe Xe Xe Xe Xe

ComplexVo (cm-1)Ro (*10-10 m)Re (*10-10 m) D2-D H2-H H2-He D2-He H2-Ne D2-Ne H2-Ar D2-Ar H2-Kr D2-Kr H2-Xe D2-Xe * All values are gas phase experimental or theoretical.