Vibrational correlations in molecular liquids

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Abstract
The theory presented in this paper investigates the vibrational correlation functions of dense molecular fluids, taking into account all vibrational transition and depopulation modes of the electronic ground state. Particular attention is paid to dynamic coupling of relevant excitations (coupled Raman bands, coupled rate equations), and to the significance of distinct vibrational correlations (i.e. correlations between adjacent molecules). Various collision induced relaxation mechanisms are included: pure dephasing arising from transition frequency fluctuations, intramolecular depopulation and resonant as well as non-resonant vibrational energy transfer. The coupling of the molecular vibrators is assumed to be weak. As a result the following conclusions apply. First the relaxation constants of the depopulation modes need not be given by the vibrational exciton annihilation rates. Second, weakly-separated or overlapping transition bands can show a significant dynamic coupling. Third, distinct vibrational correlations, although negligible at time zero, can be significant at finite times, as a result of resonant and non-resonant vibrational transfer processes. Fourth, the contribution of the depopulation process on the dephasing constant of a collective μ → v transition cannot be related to the annihilation rates of the involved vibrational excitons, μ and v.