The method of time-resolved optoacoustics was used to measure the rate of vibrational
relaxation of CH3Cl(ν6) and CH3Br(ν6) by Ar. The pressure pulses generated by the
relaxing gas revealed that the rate of production of translational energy from ν6 = 1 is
approximately twice the decay rate of IR fluorescence from
ν3 = 1. No evidence was
found for a previously proposed bottleneck in rotational relaxation, which would have
resulted in an acoustic relaxation rate slower than the fluorescence decay. The faster
rates observed here can be explained qualitatively by a rapid energy release from energy levels above ν3 which precedes the IR fluorescence. A simple three-level model,
however, is unable to explain our observations quantitatively.