Overtone fluorescence as a probe of intermode energy flow in laser pumped molecules

Abstract

Laser induced fluorescence from the overtone 2ν3 of the pumped ν3 C–F stretch fundamental in CH3F has been observed. The risetime of this fluorescence, which corresponds to equilibration of ν3 and 2ν3 by a resonant collision process, is very fast. The fluorescence decay, on the other hand, reflects loss of population from ν3 into the other CH3F modes since on the time scale of the decay, ν3 and 2ν3 are tighty coupled. Specifically, three decay rates have been observed, the slowest of which is simple vibration–translation/rotation relaxation. The two fast decay rates are controlled largely by direct vibration to vibration energy transfer processes coupling the ν3, ν6 and ν2, ν5 levels. The measurement of these two V–V rates provides two internal rate constants which describe the collisional coupling between ν3 and ν6 and between ν6 and ν2, ν5. The breathing sphere model predicts too small a value for the probability of ν3→ν6 transfer and too large a value for the ν6→ν2, ν5 transfer.