Fluorescence of supercooled molecules as a probe for intramolecular vibrational redistribution rates

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Abstract
A theory is developed for the fluorescence spectra of supercooled polyatomic molecules in a jet. A single molecular mode (the ’’system’’) is assumed to be optically active and the other modes constitute a ’’bath’’ which induces line broadening and relaxation. Since the bath is finite and is initially cold, its motion is strongly correlated with that of the system. The correlated reduced dynamics of the system and bath is treated using the Mori projection operator technique. The present ’’tetradic reduced description’’ becomes useful especially in large statistical molecules where many molecular states are involved in the fluorescence. This is precisely the limit where the effective Hamiltonian formalism commonly used for the intramolecular dynamics in intermediate size molecules becomes very tedious and impractical. The fluorescence spectra in large statistical molecules may be interpreted in terms of simple rate equations and may be used to directly monitor the intramolecular vibrational redistribution of energy (IVR). We further compare the spectra predicted for statistical and intermediate size molecules with the existing experimental results in supersonic beams.