Laser excitation and equilibration of excited vibrational states in CH ₃F

CH₃F, when its v₂ mode is pumped by the P(20) line of the 9.6-μ band of a COt Q-switch laser, exhibits fluorescence from the v₁ and v₄ modes at 3000 cm~'. This fluorescence has a risetime ≤5 μsec at 1 torr and a decay rate of 0.59±0.02 msec ^-1·torr^-1. The diffusion coefficient measured from the low pressure fluorescence decay curves is 0.076±0.011 cm ²-torr msec^-1. A very efficient near resonant energy transfer process is probably responsible for propagation of energy up the vibrational manifold. Similar processes may be important for laser induced chemical reactions. A square dependence of the intensity of fluorescence on incident cw laser power has been observed. This appears to arise mainly from population changes caused by a general temperature increase in the gas which occurs after absorption of laser energy and equilibration of all degrees of freedom of the molecule.