Classical trajectory study of collision induced intramolecular energy transfer in trans-stilbene

Previous classical trajectory simulations of a non-rotating isolated trans-stilbene molecule in its Si excited state provided evidence of a bottleneck that hinders energy how from the Franck-Condon active mode v(25) to the trans-->cis isomerisation coordinate (K. Bolton and S. Nordholm, Chem. Phys. 203 (1996) 101). In the present work we investigate this bottleneck with respect to the influence of collisions with argon atoms in the surrounding buffer gas. The aim is to obtain mechanistic understanding that will form the basis of an extended RRKM theory which can explain the experimental isomerisation rates. A realistic potential surface which reproduces the equilibrium structure and vibrational frequencies of the excited trans-stilbene molecule is used in combination with an intermolecular potential consisting of pairwise atom-atom potentials of Lennard-Jones type. Repeated collisions are simulated and the effect on the intramolecular energy flow and, in particular, on the v(25-49) bottleneck is recorded for varying gas temperature. A search for other robust subgroups of energy conserving vibrational modes is conducted. The results show that rates of intramolecular energy flow are significantly enhanced by the collisions with argon atoms. There is a direct coupling induced by the collision as well as a secondary coupling due to the rotational excitation of the trans-stilbene molecule that is caused by the collision.