COLLISIONAL ENERGY-TRANSFER BETWEEN AR AND NORMAL AND VIBRATIONALLY AND ROTATIONALLY FROZEN INTERNALLY EXCITED BENZENE-TRAJECTORY CALCULATIONS

Quasiclasical trajectory calculations of energy transfer between an ex ited benzene molecule and an argon atom were performed. Values of aver age energy transferred per collision, [Delta E], were calculated. Thre e cases were investigated. (a) Collisions with unconstrained ''normal' ' initial conditions. (b) Collisions where the rotations of the benzen e molecule are initially ''frozen.'' (c) Collisions where the out-of-p lane vibrations of the benzene molecule are initially ''frozen.'' The distributions of [Delta E] vs collision durations and the values of [D elta E] for collisions with frozen degrees of freedom are different th an those obtained in normal collisions. This indicates the effects the se modes have on the energy transfer process. The effect of rotations was found to be the largest. This indicates the predominant role rotat ions play in the energy transfer process. The effect of out-of-plane v ibrations on the efficiency of energy transfer corroborates quantum me chanical calculations which show that out-of-plane motions are particu larly efficient in energy transfer [Clary, Berenshtein, Oref, Gilbert Faraday Discussions 102 (1995)]. One in every 800 trajectories with no rmal initial conditions was found to be a supercollision. For frozen o ut-of-plane vibration the number dropped to one in 1500 and for frozen rotations it dropped even further to one in 4000. This shows the effe ct these wide angle motions have on the production of supercollisions. An impact parameter ''window'' was created in the initial conditions which enable an enhanced production of supercollisions by a factor of 4 thus helping to create a ''bank'' of supercollisions. Analysis of th e trajectories of supercollisions in the bank shows that the condition for obtaining supercollisions are dynamic in nature. The atom approac hes the molecule perpendicularly and it is in phase with a highly exci ted out-of-plane motion anti/or is hit by a fast rotating molecule. Th is also agrees very well with the previous work quoted above. It is fo und that collisions, including supercollisions, are short lived. simil ar to 60% of all inelastically scattered collisions last less than 140 fs and the rest last less than 500 fs. The number of long lived compl ex forming collisions is negligible. (C) 1997 American Institute of Ph ysics.