H + NO2 Channels in the Photodissociation of HONO at 193.3 nm

Abstract

The H + NO2 channels in the 193.3-nm photodissociation of jet-cooled HONO have been examined by using high-n Rydberg-atom time-of-flight (HRTOF) technique. Center-of-mass (CM) translational energy distribution and energy-dependent angular distribution of the photoproducts reveal that the NO2 fragments are produced in at least three electronic states: ground X̃2A1 and excited Ã2B2 and B̃2B1 (and/or C̃2A2) states. The overall average CM product translational energy is 〈ET〉 = 0.3Eavail. NO2 fragments are highly vibrationally excited in each of these electronic states, and in particular, a long vibrational progression of NO2 bending mode in the Ã2B2 state has been observed. Branching ratios of the NO2 electronic states are estimated: X̃2A1:Ã2B2:B̃2B1/C̃2A2 ≈ 0.13:0.21:0.66. The OH bond photodissociation of HONO from the second electronically excited singlet state Β1Α‘ proceeds via multiple dissociation pathways. The H + NO2(Ã2B2) product channel is via a direct dissociation (presumably in a near-planar fragmentation geometry) and has a large translational energy release, a specific NO2 bending vibration population (indicating a significant change of the ONO angle during dissociation), and an anisotropic product angular distribution (suggesting a short excited B̃1A‘ state lifetime with respect to dissociation). The H + NO2(X̃2A1) channel could be produced from a triplet excited state (which likely has a repulsive barrier along the O−H dissociation coordinate) following intersystem crossing or from the ground state of HONO after internal conversion. The H + NO2(B̃2B1) channel requires nonadiabatic processes in a planar geometry, while in nonplanar geometries, it can be directly produced from the HONO(B̃1A‘) state, consistent with its large branching ratio.