Two-phonon resonances and bound-states in molecular crystals. I. General theory

Abstract

Using the equation of motion method one- and two-phonon Green functions for the internal modes of a molecular crystal are calculated taking into account the internal anharmonicity of the single molecule. The equations thus derived, formally equivalent to those for a lattice with an impurity, are explicitly solved. These renormalised functions exhibit, if the anharmonicity is strong enough, extra poles, outside the two-phonon continuum, which can be interpreted as bound-states.