CONTRIBUTION OF THE TAIL OF A BIEXPONENTIAL ENERGY-TRANSFER PROBABILITY-DISTRIBUTION TO THERMAL UNIMOLECULAR RATE COEFFICIENTS

Experiments and quasiclassical trajectory calculations of intermolecul ar energy transfer indicate that the energy-transfer probability distr ibution function, P(E',E) has a significant contribution from high-ene rgy collisions, sometimes denoted as supercollisions. One functional f orm of P(E',E) which is used to fit the data is a biexponential functi on with a low-energy exponential and a high-energy exponential which p rovides the high-energy tail. To assess the importance of the high-ene rgy collisions, the present work evaluates the contribution of the hig h-energy tail to the value of the unimolecular rate coefficient by ass uming model biexponential probability functions and solving the approp riate master equations. Since the strong collision part of the biexpon ential function contributes to small values of the energy exchanged, D elta E, as well, a distinction is made between the high-energy exponen tial and the tail of the probability function that represents supercol lisions. Solving a master equation with the tail only, shows that supe rcollisions, in spite of their small numbers, contribute, under certai n conditions, significantly to the values of the low pressure unimolec ular rate coefficient.