Longitudinal Modes in Hydrogen and Deuterium Halide Crystals

The observation of four bands in the Raman spectra of the internal modes of crystalline HCl, DCl, HBr, and DBr has previously been explained either by assuming a four-molecule primitive cell, which is different than the structure suggested by diffraction studies, or by assuming that two of the bands were due to longitudinal modes. We have used the experimental infrared intensities with a dipolar coupling model appropriate to the cell symmetry to compute the LO frequencies. The calculated frequencies are in excellent agreement with those observed in the hydrogen and deuterium halide crystals.