Temperature dependence of rotational relaxation of methane in the 2ν3 vibrational state by self- and nitrogen-collisions and comparison with line broadening measurements

Depopulation rates of rotational levels in the v3 = 2 vibrational state of 12CH4 are investigated by a pump-probe technique. Methane molecules are excited into selected rotational levels by tuning the pump laser to 2ν3 lines. The time evolution in population of the excited level after the pumping pulse is monitored by tuning the probe laser to a (3ν3 ← 2ν3) line corresponding to a transition with the excited rotational level as the lower level. Measurements were performed from room temperature down to 100 K in pure CH4 and in CH4-N2 mixtures. The rotational relaxation rate coefficients are given for the J = 1, A2, J = 1, E, J = 1, F2 and J = 0, F2 levels. The results are compared with the available data on line broadening coefficients. The temperature dependence of the data on N2-broadening is particularly well reproduced by the power law deduced from the results on rotational relaxation.