Knowledge Bank

A new description of the dynamics of vibrational relaxation of small molecules in condensed phases is proposed. Previous theories view the relaxation as a multiphonon. process, in which a single relatively large quantum of excitation in the intramolecular vibrational mode is lost at once as many small quanta of excitation of the intermolecular (lattice”) modes. The multiphonon decay rate is calculated via first-order perturbation theory with the assumption that the equilibrium positions of the lattice” modes simply shift as the intramolecular mode undergoes a transition. In other words, the coupling of the intramolecular mode to the lattice” modes is treated perturbatively. Our new view recognizes that the interaction of the intramolecular mode with certain local” modes of the lattice” may be quite strong- Hence, the dynamics of the complex” formed from the mixing of the intramolecular mode with the relevant local” modes is handled exactly and then the coupling of the complex” modes to the remaining lattice” modes (bath) is described perturbatively. In this view, relaxation takes place in two steps. First, energy flows from the high-frequency intramolecular mode into the low-frequency local” modes and then, by single-quantum processes, into the bath. Application of the theory to specific models for diatomic impurities in rare gas matrices is discussed. The results are compared with experiment.