Photodissociation resonances of jet-cooled NO2 at the dissociation threshold by CW-CRDS

Around 398 nm, the jet-cooled-spectrum of NO2 exhibits a well identified dissociation threshold (D-0). Combining the continuous-wave absorption-based cavity ringdown spectroscopy technique and laser induced fluorescence detection, an energy range of similar to 25 cm(-1) is analyzed at high resolution around D-0. In addition to the usual molecular transitions to long-lived energy levels, similar to 115 wider resonances are observed. The position, amplitude, and width of these resonances are determined. The resonance width spreads from similar to 0.006 cm(-1) (i. e., similar to 450 ps) to similar to 0.7 cm(-1) (similar to 4 ps) with large fluctuations. The identification of at least two ranges of resonance width versus the excess energy can be associated with the opening of the dissociation channels NO2 -> NO(X-2 Pi(1/2), v = 0, J = 1/2) + O(P-3(2)) and NO2 -> NO(X-2 Pi(1/2), v = 0, J = 3/2) + O (P-3(2)). This analysis corroborates the existence of loose transition states close to the dissociation threshold as reported previously and in agreement with the phase space theory predictions as shown by Tsuchiya's group [Miyawaki et al., J. Chem. Phys. 99, 254-264 (1993)]. The data are analyzed in the light of previously reported frequency-and time-resolved data to provide a robust determination of averaged unimolecular dissociation rate coefficients. The density of reactant levels deduced (rho(reac) similar to 11 levels/cm(-1)) is discussed versus the density of transitions, the density of resonances, and the density of vibronic levels. (C) 2015 AIP Publishing LLC.