VIBRATIONAL RELAXATION OF SMALL MOLECULES IN DENSE MEDIA
Creators:Diestler, D. J.
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Publisher:Ohio State University
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.
Author Institution: Department of Chemistry, Purdue University
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