Vibration→vibration energy transfer in methane

Abstract

Vibrational energy levels of methane in the range 2800 to 6000 cm−1 were excited by a pulsed tunable laser. Infrared fluorescence decays were observed at 294 K and analyzed to yield V→V energy transfer pathways and rates. Symmetric stretching excitation is converted to asymmetric stretching in the same molecule with a rate constant k1=(3.6±0.9) ×10−11 cm3 molecule−1 s−1 (P=0.09). The ν2 and ν4 bending vibrations are interconverted somewhat less rapidly. Stretching excitation is converted to bending with a rate constant of (0.8±0.2)×10−11 cm3 molecule−1 s−1 (P=0.022) for the stretching fundamentals and about twice that for overtone or combination levels. The rate of almost exactly resonant transfer of a vibrational quantum from one molecule to another was about (1.8±0.6)×10−11 cm3 molecule−1 s−1 (P=0.05) for transfer of a bending quantum from CH4(2ν4) and about three times less for transfer of a stretching quantum.