State-specific unimolecular reaction of NO2 just above the dissociation threshold

Abstract

Photofragment excitation (PHOFEX) spectra of NO2 are observed by monitoring the specific quantum states of a fragment NO (2Π1/2;v=0, J=0.5–6.5) in the energy region 0–160 cm−1 above the dissociation limit to NO (2Π1/2) and O (3P2). Preparation of NO2 in a quasibound eigenstate above the dissociation limit is attained by the combination of extremely cooled (∼1 K) parent NO2 in a supersonic jet and a high resolution (∼0.05 cm−1) photolysis laser. The dissociation rate constants are obtained from the peak width of PHOFEX spectra and the smallest rate constant is k=8.5×109 s−1, in the energy region where only J=0.5 of NO (2Π1/2; v=0) is produced. The observation that the rate constant increases stepwise when a new product channel J=1.5 opens implies that the transition state is a loose complex. This behavior of the rate constant is direct experimental proof of the statistical theory of the unimolecular reaction process. The product state distribution of NO fluctuates depending on the quasibound state of NO2, though the average value is consistent with the calculation by phase space theory. This state specificity of the rate constant is interpreted in terms of quantum fluctuations associated with individual quasibound eigenstates.