Infrared-active vibron bands associated with rare gas atom dopants isolated in solid parahydrogen

Abstract

We report high-resolution infrared absorption spectroscopic studies of the dopant-induced Q 1 (0) vibron band in solid parahydrogen crystals doped with low concentrations of rare gas atoms. The frequency, lineshape, and integrated absorption coefficient for the rare gas atom-induced Q 1 (0) vibron band are measured for Ne, Ar, Kr, and Xe. The observed lineshapes and peak maxima frequencies are sensitive to the H 2 vibrational dependence of the dopant-H 2 isotropic intermolecular potential. Trends observed for Ar, Kr and Xe indicate the vibrational dependence is strong enough for Xe to trap the infrared-active vibron in its first solvation shell while for Ar the vibron remains delocalized. The Ne-induced feature displays a qualitatively different lineshape which is attributed to the weak intramolecular vibrational dependence of the Ne-H 2 intermolecular potential relative to the H 2 -H 2 interaction. The lineshapes of the Ar, Kr, and Xe dopant-induced Q 1 (0) pure vibrational features agree well with recent first principles calculations. The distinguishing feature of quantum crystals, such as solid helium and solid molecular hydrogen, is the large amplitude zero-point motion of the constituent atoms or molecules about their equilibrium positions in the crystal lattice [1][2][3][4][5]. In solid parahydrogen (p-H 2 ) the rootmean-square deviation of the p-H 2 molecule from its lattice site is approximately 18% of the nearest-neighbor spacing [3]. The presence of this zero-point energy dynamically inflates the molar volume of the crystal and thus solid p-H 2 has a molar volume (23.15 cm 3 ·mol -1 ) comparable to solid argon (22.42 cm 3 ·mol -1 ) and significantly greater than solid neon (13.31 cm 3 ·mol -1 ) [4,6]. This molecular motion in solid p-H 2 influences the rates of intrinsically quantum relaxation processes such as ortho-para conversion and quantum diffusion whose rates are amplified by intimate short-range interactions [4,7]. While these quantum mechanical zero-point effects are more pronounced in low density solid helium compared to solid p-H 2 , because hydrogen molecules have vibrational and rotational degrees of freedom, quantum crystals of solid p-H 2 have excitons (vibrons and rotons) that have no analogy in solid helium and these excitons can be used to spectroscopically probe the zero-point motion of the quantum solid [8].Solid mixtures of p-H 2 doped with low concentrations of rare gas (Rg) atoms are of interest to investigate how the heavy dopant species perturbs the zero-point motion of the quantum crystal [4]. X-ray diffraction studies of solid p-H 2 doped with Ne, Ar, or Kr indicate that the molar volume of the doped solid increases and the c/a ratio of the hexagonal close-packed (hcp) lattice decreases for Ne and Ar and increases for Kr [6,[9][10][11]. The increase in molar volume for the Ne and Ar doped solids is in violation of Vegard's law which holds that a linear relation exists between the crystal lattice constant of an alloy and the concentration of the constituent elements [12]. Sinc...

Infrared spectroscopic studies of the rare gas atom perturbed S1(0) rovibron band of solid parahydrogen

“…The present spectral measurements therefore compliment previous measurements of the Rg atom induced Q 1 (0) pure vibron band of solid pH 2 for which the shift in the transition energy only depends on the intramolecular dependence of the isotropic component of the Rg-H 2 intermolecular potential [14][15][16][17]. The Rg-H 2 intermolecular potentials [18,19] are among the best characterized pair-potentials and thus these systems are ideal for testing how the bound states of solid pH 2 are perturbed by the presence of an atomic dopant.…”

“…Previous studies of the Ne-induced Q 1 (0) transition in solid pH 2 revealed that the overlap component of the induced dipole moment is very small [14]. Similarly, the overlap component of the Ne perturbed S 1 (0) transition is expected to be weak, and the transition should gain most of its intensity via quadrupolar induction with the surrounding nearest-neighbor pH 2 molecules, similar to the unperturbed S 1 (0) transition.…”

“…Simple rotational energy level calculations of S 1 (0) satellite line positions using known Rg-H 2 and H 2 -H 2 intermolecular potentials agree qualitatively with the observed spectra but suggest that more sophisticated dynamical calculations are needed to quantitatively reproduce the observed spectra. This work is part of an ongoing research effort aimed at using the induced H 2 transitions in chemically doped solid pH 2 as a means of quantifying the identity and concentration of non-IR active atomic dopants [14][15][16][17].…”

Synthesis and infrared characterization of Br–HBr and Br–DBr entrance channel complexes in solid parahydrogen

“…The Br 2 concentration can also be monitored by examining the Q 1 (0) induced region. 38,41,42 Representative spectra in the Q 1 (0) induced region (B4150 cm À1 ) are shown on the right side of Fig. 3.…”

Rotationally adiabatic pair interactions of para- and ortho-hydrogen with the halogen molecules F2, Cl2, and Br2

“…2,[9][10][11] These quantum effects also have to be considered for H 2 clusters and solids doped with other molecules. [12][13][14] For example, small pH 2 clusters show superfluid responses to roa) m.berg@fu-berlin.de b) antonioaccardi80@gmail.com c) b.paulus@fu-berlin.de d) burkhard.schmidt@fu-berlin.de tations of embedded OCS, [15][16][17] CO 2 , 18 and CO 19 molecules. Theoretical investigations rely on the respective pair potentials for the interaction of the dopant with H 2 .…”