PHOTO-DISSOCIATION RESONANCES OF JET-COOLED NO2 AT THE DISSOCIATION THRESHOLD BY CW-CRDS, CHALLENGING RRKM THEORIES

Abstract

Around 398~nm, the jet-cooled ${rm NO_{2}}$ spectrum exhibits a well identified dissociation threshold ($D_{0}$). Combining LIF detection and continuous-wave absorption-based CRDS technique a frequency range of $sim{rm 25,cm^{-1}}$ is analyzed at high resolution around $D_{0}$. In addition to the usual rovibronic transitions towards long-lived energy levels, $sim115$ wider resonances are observed. Over this energy range, the resonance widths spread from $sim{rm 0.006,cm^{-1}}$ ($sim450:{rm ps}$) to $sim{rm 0.7,cm^{-1}}$ (${rm sim{rm 4:ps}}$) with large fluctuations. At least two ranges of resonance width can be identified when increasing the excess energy. They are associated with the opening of the dissociation channels ${rm NO_{2}}rightarrow{rm NO}left(X,^{2}Pi_{1/2},,v=0,,J=1/2right)+{rm O}left(^{3}P_{2}right)$ and ${rm NO_{2}}rightarrow{rm NO}left(X,^{2}Pi_{1/2},,v=0,,J=3/2right)+{rm O}left(^{3}P_{2}right)$. Weighted mean unimolecular dissociation rate coefficients $k_{mathit{uni}}$ are calculated. The density of reactants (following the RRKM predictions) is deduced, and it will be discussed versus the density of transitions, the density of resonances and the density of vibronic levels. The data are analyzed in the light of time-resolved data previously reported. This analysis corroborates the existence of loose transition states along the reaction path close to the dissociation energy in agreement with the phase space theory predictionsfootnote{[Accpeted in J. Chem. Phys.]}