The Fundamental Infrared Absorption Band of Ortho-Enriched Solid Hydrogen

Spectra of the infrared fundamental band of solid hydrogen with orthohydrogen concentrations in the range 75 to ~99% were recorded at 1.05 and ~5 K, i.e. below and above the order–disorder phase transition which occurs at 2.8 K for pure o-H2. The effect of "impurity" para molecules in the o-H2 lattice could thus be studied with and without orientational ordering of the o-H2 molecules. The zero-phonon Q1, S1(0), and S1(1) features are of particular interest, and are interpreted in terms of travelling vibrational, rotational, and orientational excitations (vibrons, rotons, and librons). The Q branch for ~100% o-H2 in the ordered state shows a structured side band, shifted by ~6–26 cm−1 from the Q1(1) 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, centered at the Q1(1) frequency, and due to predominantly low-energy orientational transitions of interacting o-H2 molecules. The S1(0) group of maxima in the ordered state is interpreted as the superposition of the transitions, Q1(1) + S0(0) and Q1(1) + S0(0) + libron, where Q1(1) is an o-H2 vibron and S0(0) is the localized rotational transition of an impurity p-H2 molecule, the J = 2 level of which is split into three sublevels by the C3i field of the Pa3 crystal structure.