Rate coefficients for the vibrational self-relaxation of NO(X2Π, ν = 3) at temperatures down to 7 K

Infrared-ultraviolet double resonance (IRUVDR) experiments have been implemented in the super-cold environment provided by a CRESU (Cinétique de Réaction en Ecoulement Supersonique Uniforme) apparatus. This method has enabled us to measure rate coefficients for the vibrational self-relaxation of NO(X2Π; ν = 3), i.e. NO(ν = 3) + NO → NO(ν < 3) + NO. Using different Laval nozzles, results have been obtained at six different temperatures between 85 and 7 K. The rate coefficients increase strongly as the temperature is lowered. The endothermicity of single quantum vibrational-vibrational (V-V) energy exchange (δE/hc = 55.9 cm−1) means that the rate of this process must decrease markedly at the lowest temperatures of our experiments. Therefore, the high relaxation rates which are observed must be due to vibrational-translational (V-T) energy transfer. It is proposed that this process is efficient because of the transient formation of (NO)2 collision complexes in which intramolecular vibrational energy transfer occurs at rates competitive with re-dissociation of the dimer to NO(ν = 3) and NO(ν = 0).