Speed-Dependent Anisotropy Parameters in the UV Photodissociation of Ozone

Abstract

Resonance-enhanced multiphoton ionization coupled with time-of-flight product imaging has been used to study the O2(3Σg -) + O(3P j ) product channel in the UV photodissociation of ozone at photolysis wavelengths of 226, 230, 240, and 266 nm. For dissociation at 226 and 230 nm the O(3P2) fragment is produced with a strongly bimodal velocity distribution, in keeping with the previous findings of Miller et al., Syage, and Stranges et al. at photolysis wavelengths of 226 and 193 nm. At the longer dissociation wavelengths of 240 and 266 nm, the bimodal velocity distribution becomes less evident in the O(3P2) product. Anisotropy parameters have been determined as a function of the O(3P2) fragment speed. A very similar and clear speed dependence is evident at all photolysis wavelengths considered, with the anisotropy parameters rising steadily as the oxygen atom speed increases. The UV dissociation dynamics of ozone to the channel producing triplet products are discussed in light of this analysis.

The vibrational distribution of O2(X 3Σg−) produced in the photodissociation of ozone between 226 and 240 and at 266 nm

“…Angular distributions are determined by integrating over the desired speed interval for each angle. 2 Extraction of the speed distribution is achieved in an analogous manner by integrating over all angles for each speed. The speed distributions can be further transformed, using the law of conservation of momentum, into total translational energy distributions for the O 3 →O 2 (X 3 ⌺ g Ϫ )ϩO( 3 P J ) dissociation.…”

“…2,35 A molecular beam of ozone was formed by flowing helium with about 810 Torr backing pressure over ozone maintained on silica gel at a temperature of Ϫ78°C. The subsequent mixture of Ͻ1% ozone was expanded through a pulsed 250 m diam nozzle and collimated by a 500 m diam skimmer mounted about 5 mm from the nozzle orifice.…”

“…The possibility that the high vibrational excitation comes from elimination of the central oxygen atom with recombination of the end atoms to form the O 2 product is appealing, but the angular distributions of the products suggest that this is not a likely mechanism. 2 The O( 3 P J ) atoms produced in coincidence with both the O 2 (vу26) and O 2 (vϽ26) have similar high and positive anisotropy parameters, whereas elimination of the central atom, at least along a C 2v path, would produce a negative anisotropy parameter. A more likely explanation for the bimodality might be that there are multiple regions where the dissociative triplet surface leading to the O( 3 P J ) ϩO 2 ( 3 ⌺ g Ϫ ) products crosses the bound excited surface, which correlates with O( 1 D)ϩO 2 ( 1 ⌬ g ).…”

“…A more likely explanation for the bimodality might be that there are multiple regions where the dissociative triplet surface leading to the O( 3 P J ) ϩO 2 ( 3 ⌺ g Ϫ ) products crosses the bound excited surface, which correlates with O( 1 D)ϩO 2 ( 1 ⌬ g ). 2 Another possible explanation is that there is more than one excited state responsible for the absorption. In order to evaluate these possibilities, it is necessary to know how the yield of the O 2 ( 3 ⌺ g Ϫ ,vу26) product varies with wavelength.…”

The ultraviolet photodissociation of jet-cooled ClO and BrO radicals

“…Miller et al 47 and Syage 48 49 observed a similar speeddependent anisotropy in ozone photodissociation at 226 nm with slightly different anisotropy parameters of 1.31Ϯ0.20 and 0.67Ϯ0.05 for the fast and slow components, respectively. In order to minimize the effect of ozone photodissociation, the polarization of the probe beam was horizontal with respect to the flight axis for all the spectra taken.…”