INFRARED-ULTRAVIOLET DOUBLE-RESONANCE MEASUREMENTS ON THE TEMPERATURE-DEPENDENCE OF RELAXATION FROM SPECIFIC ROVIBRONIC LEVELS IN NO(X(2)PI, NU=2, J) AND (X(2)PI, NU=3, J)

Infrared-ultraviolet double-resonance experiments have been performed on NO at three temperatures, 295, 200, and 80 K, to measure rate const ants for (a) total relaxation from selected levels in the nu = 2, Omeg a = 1/2 and nu = 3, Omega = 1/2 rotational manifolds of the X(2) Pi el ectronic ground state with several collision partners (M = NO, He, Ar, H-2, N-2, CO, and CO2), and (b) vibrational self-relaxation from nu = 2 and nu = 3. NO molecules were initially prepared in selected rovibr onic levels by tuning the output from an optical parametric oscillator to lines in the (2,0) or (3,0) infrared overtone bands. Loss of popul ation from the initially excited level was observed by making time-res olved laser-induced fluorescence measurements on appropriate lines in the (2,2) and (2,3) bands of the A(2) Sigma(+)-X(2) Pi electronic syst em of NO. The thermally averaged cross sections for total rotational r elaxation are found to be essentially independent of rotational state and temperature. The light collision partners (He, H-2) are the least effective, with the molecular species (NO, N-2, CO, and CO2) rather mo re effective than Ar. The results are compared with previous directly determined values for rotational relaxation in nu = 2 and higher vibra tional levels and with cross sections inferred from measurements of li ne-broadening. It is clear that vibrational self-relaxation of NO(nu=2 ) and NO(nu=3) occurs by vibration-vibration (V-V) exchange, NO(nu) NO(nu=0) --> NO(nu=1) + NO(nu=1), at a rate which is almost independen t of temperature and which seems to be uninfluenced by the presence of spin-orbit degeneracy in, and specific attractive forces between, the NO collision partners.