The role of intermolecular potential well depths in collision-induced state changes

Abstract

A relationship is developed from two distinct theoretical approaches to correlate the rate constants kM or cross sections σM for a series of added gases M which collisionally induce a state transformation A*→B. The correlation derived from theory is where C is a constant and εA*M is the intermolecular well depth between A* and M. We observe that experimental data can be described by a related correlation where β is a constant and εMM is the well depth between pairs of M molecules. This correlation is shown to be general. It works for electronic state deactivation in atoms, intersystem crossing and internal conversion in S1 polyatomics, rotational and also vibrational relaxation in S1 polyatomics, predissociation in diatomics and polyatomics, and vibrational relaxation in a free radical as well as in a molecular ion. The theory is appropriate only when attractive forces dominate the interaction, and this seems consistent with the experimental data. The correlation thus provides a simple means to distinguish between attractive and repulsive interactions. The correlation also reveals that collision partners do not substantially modify the intrinsic S1-T mixing during collision-induced intersystem crossing.