Energy dependence of energy transfer in the collisional relaxation of vibrationally highly excited CS₂

Collisional energy relaxation of highly excited CS₂ in collisions with He and Xe is investigated by using classical trajectory methods to study the vibrational energy dependence of the average energy transfer per collision. This average energy transfer has been calculated by using two techniques: (1) direct simulation of the relaxation experiment by calculation of sequences of collisions suffered by an ensemble of hot molecules, and (2) evaluation of the energy transfer for specific internal energies by using microcanonical ensembles of excited molecules. Both methods provide energy-transfer values in good agreement with experiment and with each other. The negative of the average energy transfer per collision increases linearly with increasing CS₂ energy if the collider is He and quadratically if the collider is Xe. These results are not especially sensitive to the inter-or intramolecular potentials, but the mass difference between He and Xe plays an important role. A detailed analysis of the trajectories indicates that the light collider participates in impulsive collisions, while the heavy collider participates in "slow" collisions in which the two colliders interact strongly for several vibrational periods. The validity of several different theories of the energy dependence of the energy transfer is investigated.