Collision dynamics of intermode energy flow in laser pumped polyatomic molecules: CH3F

Abstract

Subsequent to placing excess vibrational population in the ν3C–F stretching mode of CH3F by a Q-switched CO2 laser, the time dependence of fluorescence signals from the overtone level 2ν3 and from all other vibrational fundamental levels has been monitored and analyzed. The nearly resonant, ladder climbing process CH3F(ν3)+CH3F(ν3) ⇄CH3F(2ν3)+CH3F(O)+18 cm−1 reaches steady state in about 3.3 gas kinetic collisions. The excess vibrational energy contained in the ν3 mode is transferred to the remaining CH3F modes on a longer timescale. Direct coupling exists between ν3 and ν6 and between ν6 and ν2,ν5: CH3F(ν3)+CH3F⇄CH3F(ν6)+CH3F−133 cm−1 CH3F(ν6)+CH3F⇄CH3F(ν2,ν5)+CH3F−284 cm−1. These two processes have been found to reach steady state in 30 and 120 gas kinetic collisions, respectively, in pure CH3F. No evidence for the direct coupling between ν3 and ν2,ν5 has been found. Equilibration of the C–H stretch levels ν1, ν4 with the rest of the CH3F vibrational levels via the fairly efficient ladder climbing process 2CH3F(ν2,ν5) ⇄CH3F(2ν2,2ν5)+CH3F(O)+87 cm−1 which populates the overtones 2ν2,2ν5, followed by crossover between the Fermi mixed levels 2ν5 and ν1 CH3F(2ν5)+CH3F⇄CH3F(ν1)+CH3F is consistent with the data obtained in the present experiments.