Contribution of the tail of a biexponential energy-transfer probability distribution to thermal unimolecular rate coefficients

Experiments and quasiclassical trajectory calculations of intermolecular energy transfer indicate that the energy-transfer probability distribution function, P(E',E) has a significant contribution from high-energy collisions, sometimes denoted as supercollisions. One functional form of P(E',E) which is used to fit the data is a biexponential function with a low-energy exponential and a high-energy exponential which provides the high-energy tail. To assess the importance of the high-energy collisions, the present work evaluates the contribution of the high-energy tail to the value of the unimolecular rate coefficient by assuming model biexponential probability functions and solving the appropriate master equations. Since the strong collision part of the biexponential function contributes to small values of the energy exchanged, Delta E, as well, a distinction is made between the high-energy exponential and the tail of the probability function that represents supercollisions. Solving a master equation with the tail only, shows that supercollisions, in spite of their small numbers, contribute, under certain conditions, significantly to the values of the low pressure unimolecular rate coefficient.