Theory of a phonon–exciton resonance in a molecular crystal

Abstract

The theory of a coupled phonon–roton (rotational exciton) mode in a molecular crystal is developed using the propagator technique. The dispersion relation and the phonon density of states are calculated. The results are applied to the case of solid HD. The line-shape function for the one phonon branch, phonon–roton coupling, and the roton band frequencies are calculated for this case. Near the phonon–roton resonance, the one phonon branch shows strong influence of the interaction with the roton band.