The role of van der Waals molecules in vibrational relaxation processes

Abstract

A theoretical model is presented which shows that a van der Waals molecule containing a vibrationally excited chemically bonded molecule can often undergo efficient vibrational predissociation. This predissociation occurs when the vibrational energy pours into the weak van der Waals bond causing its rupture and producing the chemically bonded molecule in its ground vibrational state. At sufficiently low temperatures where the concentration of van der Waals molecules can be significant. vibrational predissociation can dominate the vibrational relaxation process. This dominance is achieved if the time scale for establishing the equilibrium concentration of the van der Waals molecules is short compared with other relaxation processes. The kinetics of van der Waals molecule formation are discussed and the conditions for establishing equilibrium are given. When these conditions are met and at a temperature near one half the intermolecular potential well depth the vibrational relaxation rate constant will increase with decreasing temperature because of the presence of van der Waals molecules. We present calculations of vibrational predissociation lifetimes, van der Waals molecule equilibrium constants and vibrational relaxation rate constants for H 2 *-H 2, NO *-NO and other simple systems. These results are compared as far as possible with available experimental data. We have found no evidence that van der Waals molecules play an important role in the kinetics of vibrational relaxation processes. New vibrational relaxation experiments are proposed, however, which should reveal their presence.