UNIMOLECULAR FRAGMENTATION RATE THEORY REVISITED - AN IMPROVED CLASSICAL-THEORY

We propose a modification of the approximate classical theory of unimolecular fragmentation developed by Gray, Rice, and Davis; the important difference is the use a vibrational state dependent separatrix as the bottleneck to reaction. This version of the theory is well adapted to the description of the predissociation of van der Waals molecules, the rates of fragmentation of which are poorly described by the conventional statistical theory of unimolecular reaction. A test of the modified theory against molecular dynamics simulations of the predissociation of HeI2 reveals considerable improvement relative to the predictions of the Gray-Rice-Davis treatment. For large vibrational excitation, when intramolecular energy transfer is fast enough to not influence the predissociation rate, there is also very good agreement with the experimental data of Johnson, Wharton, and Levy. For small vibrational excitation it is necessary to explicitly account for the slow intramolecular transfer of energy in order to reproduce the experimental data.