Anisotropic Intermolecular Interactions in the Rotation–Vibration Raman Spectrum of Solid Hydrogen

Abstract

The rotation–vibration Raman spectra of solid mixtures of ortho- and para-hydrogen have been studied over a wide concentration range. If the proportion of J = 1 molecules is below ~ 70% the samples have an h.c.p. structure at all attainable temperatures; for higher concentrations a change to an f.c.c. structure occurs at ~ 1.3 K. In the h.c.p. phase the S1 lines show no structure, and the principal effect of anisotropic interactions is to decrease the degree of localization, and thus the energy, of the bound state into which the Raman transition takes place. This decrease is negligible for the S1(1) transition in hydrogen and at all concentrations the frequency is approximately that calculated on the basis of isotropic interactions only, so that in this phase crystalline field effects must be small.In the f.c.c. phase both lines are split into a number of components, and the structure of the S1(1) transition in hydrogen is well represented by the splitting of a bound J = 3 state by a Y20 crystalline field due to quadrupole–quadrupole interactions between nearest neighbors. In deuterium other effects of anisotropic interactions are also important.The observation of double transitions shows a dependence of the anisotropy of the polarizability of the solid on the molecular interactions.