Temperature dependence of vibrational relaxation times in gases

Abstract

Ultrasonic dispersion measurements at varying temperatures, extending over the range 290 to 580° K, have been made on gaseous ethylene, cyclo propane, carbon tetrafluoride, methyl chloride and methyl bromide. The results are correlated with previous measurements on methyl fluoride and sulphur dioxide. The non-polar gases show a steady rise in the probability of energy transfer between translation and vibration with rise in temperature. The transition probability, P 10 , is found to vary with exp — T -½ in accordance with current collision theory, but the quantitative dependence cannot be predicted from molecular properties. The polar gases behave in a similar way at higher temperatures, but at lower temperatures the transition probability increases with falling temperature. This is interpreted as due to increasing predominance of oriented collisions, which are specially favourable for energy transfer, between polar molecules at lower temperatures.