The transfer of vibrational energy between molecules

Abstract

Ultrasonic dispersion measurements have been made in mixtures of SF 6 + CHClF 2 , extending over the whole concentration range. All mixtures showed a single dispersion zone corresponding to relaxation of the whole of the vibrational energy of both components, with approximately quadratic dependence of reciprocal relaxation time on molar concentration. A quantitative interpretation is based on the assumption that the vibrational energy of the whole system is maintained in continuous equilibrium by rapid near-resonant transfer of vibrational energy between the 344 cm -1 mode of SF 6 and the 369 cm -1 mode of CHClF 2 . The collision number for this transfer is estimated as Z AB ~ 50, which is lower than the collision numbers for homomolecular deactivation of SF 6 (Z 10 = 1005) and CHClF 2 (Z 10 = 122). This behaviour is contrasted with that of mixtures previously investigated, which showed double dispersion with linear concentration dependence of reciprocal relaxation times. A quantitative reinterpretation of these is based on the assumption of a near-resonant vibration-vibration transfer, with a rate intermediate between the homomolecular deactivation rates of the two pure components. Values of Z AB ranging from 5 to 110 are estimated for these systems. The significance of the available values of Z AB for vibration-vibration transfer is discussed, and their general pattern, in relation to the vibrational frequencies of the exchanging modes, is shown to be in accord with the quantum-mechanical theory of Schwartz, Slawsky & Herzfeld.