Vibrational energy transfer in methane and methane–rare-gas mixtures

Abstract

The vibrational relaxation time for the asymmetric stretch ν3 of CH4 is 3.9±0.6 μsec torr in pure CH4 and 12, 13, 10, 5.9, 5.5, 5.0, 11, and 9.5 μsec torr (±20%) for CH4 dilute in 3He, 4He, Ne, Ar, Kr, Xe, H2, and HD, respectively, at 295°K. The ν3 fundamental was pumped by a tunable infrared laser. The rise time for bending fluorescence following ν3 excitation is 30%–50% shorter than the ν3 decay. In pure CH4, measurements of emission from 2ν4 and ν2+ν4 indicate that a large fraction of the vibrational energy is channeled through these two levels. The main relaxation path appears to be transfer from ν3 through 2ν4 to ν4, with the ν2 bending vibration being excited mostly in the later stages of V→V transfer.