State-resolved collisional energy transfer in highly excited NO2.: II.: Vibrational energy transfer in the presence of strong chemical interaction

The state-resolved collisional self-relaxation of highly (optically) excited NO2 (E-int approximate to 18 000 cm(-1)) in a thermal cell has been probed directly using time-resolved optical double resonance spectroscopy. The thermally averaged state-to-state cross sections have been derived from a master equation analysis of the kinetic traces. Rovibrational energy transfer (intramolecular V-V, V-T,R) was found to be more than an order of magnitude less efficient than pure rotational energy transfer (R-T, R-RT) within a vibrational state. The obtained cross sections for vibrational energy transfer are discussed with respect to the different relaxation mechanisms of the molecule, i.e., direct "fast'' relaxation NO2(nu(i)) + NO2-->NO2(nu(f)) + NO2 and complex forming collisions NO2(nu(i)) + NO2-->N2O4-->NO2(nu(f)) + NO2, and compared with high pressure recombination rates k(infinity). The experiments show that the observed collisions are closer to the impulsive than to the complex forming limit. In addition, we have discussed the magnitude of the experimental relaxation rates in terms of excited state couplings and the influence of vibronic chaos on the relaxation of highly excited NO2. (C) 1999 American Institute of Physics. [S0021-9606(99)00402-X].