Vibrational Relaxation in Liquids

Attempts have been made to apply the binary collision theory of vibrational deactivation to the liquid phase. The change of collision frequency with density was approximately accounted for, but the transition probability per collision was assumed independent of density. This latter assumption is investigated here by an addition, to the direct intermolecular force, of the frictional and random forces of Brownian motion theory. It is found that the frictional force has negligible effect, but the random force makes low‐energy collisions much more effective in causing transitions than is predicted for an isolated binary collision by the semiclassical theory of Zener, Landau, and Teller. As a consequence, the isolated binary collision theory cannot be applied to liquids although it should be adequate for fairly dense gases. Probably the Brownian motion theory exaggerates the neighbor effects in liquids; the point is discussed.