Structural and quantum-diffusion studies in the near-infrared-spectra of solid hydrogen

Abstract

Experimental data on the infrared spectra of the fundamental and first overtone bands of nearly pure solid orthohydrogen, of the fundamental, first, and second overtone bands of nearly pure solid parahydrogen, are applied to analyses of the energy levels of solid hydrogen and quantum diffusion. Spectra of the fundamental band of solid hydrogen with orthohydrogen concentrations in the range 75% to ~ 99% orthohydrogen were recorded at 1.05 K and ~ 5 K, i.e., above and below the order-disorder phase transition. The zero-phonon features are interpreted in terms of travelling vibrational, rotational, and orientational excitations (vibrons, rotons, and librons). The Q branches (fundamental and overtone) for ~ 100% o-H2 in the ordered state show a structured side-band, shifted by ~ 6 - 26 cm-1 from the appropriate vibrational transition frequency and due to one- and two-libron excitations; in the disordered state, this becomes a Boltzmann-modified band of half-width ~ 12 cm-1, peaked at the vibrational frequency, and due to predominantly low-energy orientational transitions of interacting o-H2 molecules. The Si(0) group of maxima in the ordered state is interpreted as the superposition of the transitions, Qi (1) + S0(O) and Qi (1) + S0 (0) + libron, where Qi (1) is an o-H2 vibron and S 0(O) is the localized rotational transition of an impurity p-H2 molecule, the J = 2 level of which is split into three sublevels by the field of the Pa3 crystal structure. Spectra of the first and second overtone bands of pure and nearly pure solid p-H2 were recorded. A number of features were observed for the first time and are interpreted in terms of the theory of Van Kranendonk. In particular, this work marks the first investigation of the second overtone region where four zero-phonon features were observed, all due to double transitions. The fundamental band of almost pure p-H2 solid was investigated for evidence of the diffusion of o-H2 impurity molecules. The absorption features in this band which are due to interacting pairs of o-H2 molecules was observed to increase as a function of time. This growth is characterized by a rate constant which has been measured as a function of temperature in the range 1.15 K to 2.10 K and concentration in the range 0.7% to 1.6% o-H2 impurity. The value and concentration dependence of the rate constant are found to be in generally good agreement with the theory of Oyarzun and Van Kranendonk. Measurements have also been made which yield the ratio of the random number of impurity pairs to the equilibrium number as a function of temperature and concentration. These data are compared with a statistical model of the diffusion process.