Collisional energy transfer, intramolecular vibrational relaxation and unimolecular reactions

Abstract

Experiments on (a) association reactions at very high pressures and (b) vibrational relaxation between species capable of forming a strong chemical bond have confirmed the hypothesis that, for the same chemical system, these two rates are essentially the same, both reflecting the rate of formation of collision complexes. The results of these measurements are reviewed. Subsequently, cases are discussed where the attraction between the collision partners is less strong. In these cases, it seems that the vibrational relaxation rate corresponds to a lower limit for complex formation, as complexes formed in collisions involving a vibrationally excited species may redissociate before intramolecular vibrational relaxation within the complex randomises the energy located in the initially excited vibration. A number of examples, CH+N₂, NO+NO₂, NO+NO, several ion–molecule systems and several involving OH and unsaturated molecules, where this behaviour seems to be occurring are discussed. It is emphasised that, whether or not the limiting, ‘strong attraction’, behaviour is observed will depend on the energy of the colliding species and therefore, in thermal experiments, on temperature. Finally, some consideration is given to the state-to-state aspects of vibrational energy transfer in collisions proceeding via collision complexes.