Mode specific vibrational energy transfer in CH₃F-N₂O mixtures

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-N ₂O 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.