Kinetics and mechanism of the vibrational relaxation of NOX²Π(v= 1) in the temperature range 100–433 K

The first vibrational level of NOX²Π was excited with an intense light flash and relaxation times were measured from 100 to 433 K. At 210 K and below, the rate equation is –d[NO(ν= 1)]/dt=[NO(ν= 1)](k₁[NO]+k₂[NO][M]). k₁ is the rate coefficient for self-relaxation in bimolecular collisions and varies inversely with temperature. The term k₂[NO][M] corresponds to vibrational relaxation in ternary collisions.

The negative temperature coefficient for relaxation in bimolecular collisions was interpreted qualitatively by postulating that the potential energy of the resonance for an electronic-vibrational transition is orientation dependent; the lowest resonance potential may correspond to the most stable configuration of the (NO)₂ dimer. The role of ternary collisions is probably to remove NOX²Π(ν= 1) as stabilized dimer. From the dissociation constants of the dimer, rates of bimolecular dissociation were computed assuming that k₂ corresponds to the rate coefficient for dimerization. The magnitudes of the bimolecular rate coefficients were consistent with the energy transfer model, provided all degrees of freedom can contribute efficiently to bond rupture. Rate coefficients for vibrational exchange from NO to N₂ were measured at 298 and 433 K. Evidence for “vibrational equilibrium” was found in experiments with NO + CO mixtures at low temperature.