The influence of local electronic character and nonadiabaticity in the photodissociation of nitric acid at 193 nm

Abstract

The dissociation of nitric acid upon nb,O →* NO 2 excitation at 193 nm has been studied in a crossed laser-molecular beam apparatus. The primary reaction channels are OHϩNO 2 and OϩHONO. We measure the branching ratio between these two competing processes and determine ͑OHϩNO 2 ͒/͑OϩHONO͒ϭ0.50Ϯ0.05. Our experiments provide evidence of a minor OϩHONO pathway, which we assign to O(3 P) and HONO in its lowest triplet state. The dominant pathway correlates to O(1 D)ϩHONO(X 1 AЈ). The translational energy distributions reveal two distinct pathways for the OHϩNO 2 channel. One pathway produces stable NO 2 fragments in the 1 2 B 2 electronic state. The second pathway produces unstable NO 2 fragments which undergo secondary dissociation to NOϩO. We examine the influence of nonadiabaticity along the OHϩNO 2 reaction coordinate in order to explain the significant branching to this other channel. Finally, we introduce a new method for generating correlation diagrams for systems with electronic transitions localized on one moiety, in which we restrict the changes allowed in remote molecular orbitals along the reaction coordinate. Analysis of previously measured XϩNO 2 photofragment pathways in nitromethane and methyl nitrate provides further support for using a restricted correlation diagram to predict the adiabatic and nonadiabatic product channels.

An ab initio and dynamics study of the photodissociation of nitric acid HNO3

“…Furthermore, the position of the FC point where the wave packet is initially created, let us surmise that after creation, the moving wave packet is subject to a bifurcation. This expectation was justified as is demonstrated by the wave packet evolution in [8,9] leads to OH(X) + NO 2 (1 2 B 2 ) corroborating the previous analysis of Myers et al [9].…”

“…Photolysis at 193 nm is dominated by the formation of O + HONO [8,9] with a total quantum yield of 0.67 (Ø = 0.54 and 0.13 for channel (3) and (4), respectively [9] and Ø = 0.33 for (1) and (2)). This clear preference of the breaking of the strong -N=O bond (~ 500 kJ/mol) over that of the much weaker -N-O bond (~200 kJ/mol) at higher excitation energy is somewhat surprising and let to speculations about the mechanistic features of this branching process [2].…”

Evaluated kinetic and photochemical data for atmospheric chemistry: Volume I - gas phase reactions of O<sub>x</sub>, HO<sub>x</sub>, NO<sub>x</sub> and SO<sub>x</sub> species

“…A bimodal rotational state distribution was observed for HO, consistent with the bimodal translational distribution found for HO by Myers et al (1997). An additional decay channel yielding O( 3 P) was observed and attributed to channel (2) …”

“…The results of Turnipseed et al (1992) are therefore preferred and are the basis of our preferred values for the quantum yields at 193 nm. The value φ(HO) obtained by Turnipseed et al (1992) is the same as that from the molecular beam study of Myers et al (1997), and the value of φ[O( 3 P)+O( 1 D)] of Turnipseed et al (1992) can also be reconciled with the molecular beam results when production of O atoms from dissociation of internally excited NO 2 from channel (1) is allowed for. However the value of φ[O( 1 D)] = 0.28 obtained by Turnipseed et al (1992) is much smaller than the value of φ 5 = 0.52 found by Myers et al (1997), and this discrepancy remains to be resolved.…”

“…The photolysis of HONO 2 at 193 nm has been clarified by the recent molecular beam studies of Myers et al (1997) and , which employed LIF and TOF techniques to detect the photolysis products and to measure their energy distributions. They obtained evidence for the occurrence of channels (1), (2), (3), (5), and (6), with O atom production becoming a major process at this wavelength, directly through channels (2), (5), and (6), and indirectly through dissociation of the internally excited NO 2 produced via channel (1).…”