Vibrational Relaxation of Methane

When methane gas is heated in an incident shock wave, its absorption of the 1470-Å xenon resonance radiation increases to a higher equilibrium value at a rate which is associated with the rate of vibrational relaxation. Some values of the absorption coefficient k from I/I0= exp(—kx) at thermal equilibrium taken from a smoothed curve are: 1.2 cm−1 at 650°K, 10 at 1000, 47 at 1500, and 95 at 2000. In the temperature range from 740° to 1600°K, the vibrational relaxation time of a 1:9 methane—argon mixture was observed to be exp(−5.8+55 T−⅓) μsec·atm, and it was exp(−5.4+45 T−⅓) μsec·atm for a 1:1 methane—argon mixture. From these results, a relaxation time of exp(−5.4+40 T−⅓) μsec·atm was calculated for pure methane, which agrees with room-temperature ultrasonic data, and a relaxation time of exp(−6.2+61 T−⅓) μsec·atm was calculated for a trace of methane in argon. A comparison is made with data for the relaxation time of oxygen containing small amounts of methane.