Theory of the Phonon Branches in the Infrared Spectrum of Solid Hydrogen

The effect of the lattice vibrations on the infrared absorption spectrum of solid hydrogen is investigated theoretically. General expressions are derived for the integrated intensities of the phonon branches in the rotational and vibrational spectrum of solid parahydrogen. It is shown that the intensities of the pure rotational and vibrational absorption features are not appreciably affected by the lattice vibrations. A general criterion is derived for the vanishing of the contribution of instantaneous phonon processes to the integrated intensities of the phonon branches. The integrated intensities are expressed in terms of the mean square displacements of the molecules from their equilibrium positions. Explicit calculations are presented, based on the Einstein and Debye models for the lattice vibrations, and the results are compared with the available experimental data.