Photodissociation of Vibrationally Excited Pernitric Acid: HO2NO2 (2ν1) + 390 nm

Abstract

Pernitric acid (HO2NO2) molecule vibrationally excited in the vicinity of its first OH stretching overtone (2ν1) is dissociated via electronic excitation using 390-nm light. The nascent energy distribution of the resulting OH fragments is probed by using laser-induced fluorescence at sub-Doppler resolution. We find that for the minor OH + NO3 channel, 55% of the ∼19 600 cm-1of available energy goes into relative translation of the two fragments and 45% into internal excitation. In addition, electronically excited NO2 associated with the dominant HO2 + NO2 channel is also observed. The visible fluorescence from the excited NO2 fragments is sufficiently intense to allow vibrational action spectra of HO2NO2 to be recorded with good signal-to-noise. As vibrational states in the vicinity of the 2ν1 level can also undergo unimolecular dissociation on the ground electronic surface, comparing relative integrated intensities of spectral features appearing in the vibrational state selected action spectra with their known total infrared absorption cross sections provides a means for estimating unimolecular dissociation quantum yields from these levels under collision-free conditions. The present results indicate that at 298 K the unimolecular dissociation quantum yield for the HO2NO2 (2ν1) state is ∼30 ± 5%. Evidence is also presented for the generation of HOONO impurity in the standard synthesis route for HO2NO2.