Ultrasonic dispersion in halo-ethylene vapours

Abstract

The velocity and absorption of ultrasonic waves have been measured by acoustic interferometer in the vapours of vinyl fluoride, chloride, bromide and iodide, vinylidene fluoride, cis -dichloro-ethylene, trans -dichloro-ethylene, trichloro-ethylene and tetrafluoro-ethylene at 100° C and for values of f|p ranging from 100 kc s -1 atm -1 to 15 Mc s ~1 atm~1. All show dispersion or incipient dispersion, and single relaxation times appear to control the whole of the molecular vibrational energy in each case. The results are correlated with previous results for halo-methane vapours and for other polyatomic molecules. The conclusion is drawn that vibrational activation enters a molecule via the mode of lowest frequency. The probability of excitation of that mode in collision is a function both of its frequency and of the intensity of its infra-red activity. In most cases a strongly infra-red active mode is very much more easily excited by collision than an inactive mode of the same frequency. The paper includes second virial coefficient data for all the vapours investigated.