Are vibrational relaxation times really constant? II. The vibrational relaxation of C 2H 2

Abstract

Vibrational relaxation times of pure acetylene in the gas phase were measured behind incident shock waves in the temperature range 613–1184 K using a laser-schlieren technique. Overall, the results are in excellent agreement with those of acoustic and laser excitations. However, we find a marked intrinsic time dependence of the phenomenological time, which varies by factors of two to three over a wide dynamic time scale of at least six natural lifetimes. In other words, the Bethe—Teller law fails. This is confirmed by numerical solution of the master equation for a wide choice of intermode collisional coupling parameters. The density of states involved in the energy transfer process determines whether the relaxation time increases or decreases with time, and the effect is amplified by the importance of intermolecular VV processes relative to intramolecular processes.