Two-particle vibrational excitations in molecular crystals

In this paper we utilize a two-particle band model to describe cooperative vibrational excitations and vibrational overtones in molecular crystals of diatomic molecules. The cooperative vibrational excitation involves a two-vibron continuum, while the overtone corresponds to a two-vibron bound state. The problem of two-particle vibrational excitations was shown to be equivalent to the perturbation of a band by a local potential, whose strength is determined by the intramolecular anharmonicity. Model calculations were performed for a three-dimensional, semicircular density of states, resulting in transparent expressions for the energetics and intensities, which were utilized subsequently to provide a self-consistent analysis of the available experimental results. In all these cases, the local perturbation is sufficiently strong to ensure the appearance of a bound overtone state outside the two-particle band. Finally, we consider the effects of structural disorder on two-vibron states, which provide a diagnostic tool for the identification of two-particle bound states and of two-particle continua.