The vibrationally mediated photodissociation dynamics of nitric acid

Abstract

Vibrationally mediated photodissociation, in which one photon prepares a highly vibrationally excited molecule by vibrational overtone excitation and a second photon dissociates the vibrationally excited molecule, is a means of studying the spectroscopy and photodissociation dynamics of highly vibrationally excited states. Applying this dissociation scheme to nitric acid (HONO₂) excited in the region of the third overtone of the O–H stretching vibration (4ν_OH) and detecting the OH fragment by laser induced fluorescence determines the energy partitioning and identifies the influence of vibrational excitation prior to dissociation. Vibrationally mediated photodissociation using 755 and 355 nm photons deposits more energy in relative translation than the isoenergetic single photon dissociation with 241 nm light. The former process also produces three times more vibrationally excited OH fragments, and both processes form electronically excited NO₂, which receives over three‐quarters of the available energy. In these experiments, vibrational overtone excitation enhances the cross section for the electronic transition by about three orders of magnitude. The observed differences are consistent with the motion of the vibrationally excited molecule on the ground electronic state surface strongly influencing the dissociation dynamics by allowing access to different electronic states in the photolysis step.