TRAJECTORY CALCULATIONS OF RELATIVE CENTER-OF-MASS VELOCITIES IN COLLISIONS BETWEEN AR AND TOLUENE

Average velocities of Ar relative to the center of mass of toluene in bimolecular collisions were determined using quasiclassical trajectory calculations. The collision durations were binned in 20 fs and 100 fs bins and for each bin the velocities of all trajectories were average d. 10000 trajectories were calculated. About 64% of all collisions wer e elastic and the rest were inelastic collisions. The remaining 36% in elastic collisions can be classified into four types. (a) Impulsive co llisions of duration 0-300 fs (62%). (b) Chattering collisions of dura tion longer than 300 fs but Shorter than intramolecular vibrational re laxation (IVR) times (>30%). (c) Complex forming collisions which last longer than molecular IVR times but less than complex (molecular+tran sition modes) IVR times and complex forming collisions which last long er than complex IVR times. The latter may lead to statistical distribu tion of energy in the collision complex. These long lived trajectories have negligible contribution to the value of the average energy trans ferred. (d) Supercollisions (0.12%) which are collisions which transfe r an inordinate amount of energy in one event. The details of the coll isional process are discussed and sample distributions are presented. Analysis of the collision events indicate that out-of-plane vibrations and overall rotations play a major role in the energy transfer mechan ism. A comparison with existing analytical energy transfer models is p resented and it is shown that some of them do not agree with the prese nt trajectory calculation results. It is suggested that supercollision s in the gas phase and in solution play a major role in chemical react ions. (C) 1996 American Institute of Physics.