Infrared-ultraviolet double-resonance measurements on the temperature dependence of relaxation from specific rovibronic levels in NO(X2Π, v = 2, J) and (X2Π, v = 3, J)

Infrared-ultraviolet double-resonance experiments have been performed on NO at three temperatures, 295, 200, and 80 K, to measure rate constants for (a) total relaxation from selected levels in the v = 2, Ω = 1/2 and v = 3, Ω = 1/2 rotational manifolds of the X²Π electronic ground state with several collision partners (M = NO, He, Ar, H₂, N₂, CO, and CO₂), and (b) vibrational self-relaxation from v = 2 and v = 3. NO molecules were initially prepared in selected rovibronic levels by tuning the output from an optical parametric oscillator to lines in the (2,0) or (3,0) infrared overtone bands. Loss of population from the initially excited level was observed by making time-resolved laser-induced fluorescence measurements on appropriate lines in the (2,2) and (2,3) bands of the A²Σ⁺-X²Π electronic system of NO. The thermally averaged cross sections for total rotational relaxation are found to be essentially independent of rotational state and temperature. The light collision partners (He, H₂) are the least effective, with the molecular species (NO, N₂, CO, and CO₂) rather more effective than Ar. The results are compared with previous directly determined values for rotational relaxation in v = 2 and higher vibrational levels and with cross sections inferred from measurements of line-broadening. It is clear that vibrational self-relaxation of NO(v=2) and NO(v=3) occurs by vibration-vibration (V-V) exchange, NO(v) + NO(v=0) → NO(v-1 ) + NO(v=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 specific attractive forces between, the NO collision partners.