Mode specific vibrational energy transfer in CH3F–N2O mixtures

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Abstract
Fluorescence signals have been observed from the symmetric stretching and bending fundamentals of N2O in CH3F–N2O mixtures following excitation of CH3F by a Q-switched CO2 laser. The rise rates of the N2O fluorescence signals show that energy is transferred selectively from CH3F to the symmetric stretch of N2O despite the fact that overtones of the N2O bending mode are more nearly in resonance with CH3F states expected to be involved in the energy transfer process. Vibrational energy transfer studies of the same two modes in pure N2O, in which the symmetric stretch is depleted by N2O laser radiation, provide independent measurements of rate constants necessary for a kinetic analysis of the CH3F–N2O results. In pure N2O a rate of 28±1.5 msec−1 Torr−1 is assigned to equilibration of the symmetric stretching fundamental with a component of the overtone of the bending mode. A mechanism for energy transfer in CH3F–N2O mixtures is proposed which explains the important features of the experimental observations in this system and is consistent with the results in pure N2O. This model allows an upper limit to be placed on the rate constant for direct energy transfer from CH3F to the bending mode manifold of N2O, and therefore a lower limit for the actual specificity of intermolecular energy transfer is determined.