Infrared-ultraviolet double resonance measurements on the temperature dependence of rotational and vibrational self-relaxation of NO(X 2Π, ν = 2, j)

Infrared-ultraviolet double resonance experiments have been performed to measure the rates of rotational and vibrational self-relaxation in NO at three temperatures: 295 K, 200 K, and 77 K. Pulses of tunable infrared radiation from an optical parameteric oscillator have been used to excite molecules into selected rotational levels (j = 0.5, 6.5, or 15.5) in the |ν = 2; Ω = 1/2) vibronic component of the X²Π electronic ground state of NO. Loss of population from the initially excited level was observed by making time-resolved laser-induced fluorescence measurements on appropriate lines in the A²σ⁺ - X²Π(2,2) band. The rate constants for removal of population from specific rovibronic levels are essentially independent of j and at 295 K agree well with previous direct measurements on a range of ν, j levels. The rotationally thermalized population in ν = 2 relaxes by vibration-vibration (V-V) energy exchange, NO(ν = 2) + NO(ν = 0) → 2 NO(ν = 1 ), at a rate which is almost independent of temperature and which seems to be uninfluenced by the presence of spin-orbit degeneracy in, and attractive forces between, the NO collision partners.