The transfer of vibrational energy between molecules

Abstract

Abstract Ultrasonic dispersion measurements have been made in mixtures of SF6 + CHClF2, 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 SF6 and the 369 cm-1 mode of CHClF2. The collision number for this transfer is estimated as ZAB ~ 50, which is lower than the collision numbers for homomolecular deactivation of SF6(Z10 = 1005) and CHClF2 (Z10 = 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 ZAB ranging from 5 to 110 are estimated for these systems. The significance of the available values of ZAB 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.