RATE COEFFICIENTS FOR THE VIBRATIONAL SELF-RELAXATION OF NO(X-2-PI, V=3) AT TEMPERATURES DOWN TO 7 K

Infrared-ultraviolet double resonance (IRUVDR) experiments have been i mplemented in the super-cold environment provided by a CRESU (Cinetiqu e de Reaction en Ecoulement Supersonique Uniforme) apparatus. This met hod has enabled us to measure rate coefficients for the vibrational se lf-relaxation of NO(X (2) Pi; upsilon = 3), i.e. NO(upsilon = 3) + NO --> NO(upsilon < 3) + NO. Using different Laval nozzles, results have been obtained at six different temperatures between 85 and 7 K. The ra te coefficients increase strongly as the temperature is lowered. The e ndothermicity of single quantum vibrational-vibrational (V-V) energy e xchange (Delta E/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 t o 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 trans fer occurs at rates competitive with re-dissociation of the dimer to N O(upsilon = 3) and NO(upsilon = 0). (C) 1997 Elsevier Science B.V.