Raman-Active Optical Phonons in the Hexagonal Phases of SolidH2,D2, and HD

Abstract

Raman scattering from the transverse optic phonon in the hcp phase of single crystals of the five modifications of solid hydrogen has been observed. Frequencies have been measured to a precision of 0.05 ${\mathrm{cm}}^{-1\mathrm{}}$ and are compared with results of recent quantum-crystal theories of lattice dynamics. Reasonable agreement with theory is found, although frequency differences between ortho and para modifications of the same isotope are much larger than predicted. Absolute measurements of the phonon scattering efficiencies are made by comparing to the intensities of the $J=1\mathrm{}\to 3 or 0\to 2$ rotational Raman lines. The results are in good agreement with Werthamer's recent theory for light scattering in van der Waals-type crystals. A broad continuous feature observed at shifts of 40-200 ${\mathrm{cm}}^{-1\mathrm{}}$ is tentatively identified as a two-phonon band. Predictions are made for the phonon intensities expected in solid hcp helium.