We report the results of calculation of the intramolecular vibrational relaxation (IVR) rate of an isolated polyatomic molecule in a broad range of energies. The energy dependence of the relaxation rate was found to feature threshold properties: the relaxation rate was zero below some critical energy Ecand non-zero at higher energies. We believe that such a threshold behavior can be associated with the development of a dynamic chaos in the system. Calculations were made by the method of Green functions with the use of a self-consistent quasiharmonic resonance approximation. We obtained a closed set of non-linear algebraic equations for quasiharmonic frequencies and relaxation rates of the molecular vibrational modes, and numerically analyzed the solutions of the set for molecules with 9 and 18 degrees of freedom. The numerical results obtained for relaxation rates agree well with the available experimental data. We demonstrate here the possibility of the existence of highly excited long-lived states in some systems. © 1986.
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