STATE-SPECIFIC UNIMOLECULAR REACTION OF NO2 JUST ABOVE THE DISSOCIATION THRESHOLD

Photofragment excitation (PHOFEX) spectra of NO2 are observed by monit oring the specific quantum states of a fragment NO (2PI1/2;v = 0, J = 0.5-6.5) in the energy region 0-160 cm-1 above the dissociation limit to NO (2PI1/2) and O (3P2). Preparation of NO2 in a quasibound eigenst ate above the dissociation limit is attained by the combination of ext remely cooled (approximately 1 K) parent NO2 in a supersonic jet and a high resolution (approximately 0.05 cm-1 ) photolysis laser. The diss ociation rate constants are obtained from the peak width of PHOFEX spe ctra and the smallest rate constant is k = 8.5 X 10(9) s-1, in the ene rgy region where only J = 0. 5 of NO (2PI1/2; v = 0) is produced. The observation that the rate constant increases stepwise when a new produ ct channel J = 1.5 opens implies that the transition state is a loose complex. This behavior of the rate constant is direct experimental pro of of the statistical theory of the unimolecular reaction process. The product state distribution of NO fluctuates depending on the quasibou nd state of NO2, though the average value is consistent with the calcu lation by phase space theory. This state specificity of the rate const ant is interpreted in terms of quantum fluctuations associated with in dividual quasibound eigenstates.