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.