Three-Body Dissociation Dynamics of Excited States of O 3 (D 2 O) A. K. Luong, T. G. Clements, and R. E. Continetti* Department of Chemistry and Biochemistry, UniVersity of California, San Diego, 9500 Gilman DriVe, La Jolla, California 92093-0314 ReceiVed: June 15, 1999; In Final Form: August 17, 1999 In these experiments, the dynamics of the three-body dissociative photodetachment of O 3 - (D 2 O) at 258 nm are directly probed for the first time. Photodetachment of a negatively charged precursor, with coincident energy analysis of the photoelectron, allows production of energy-selected excited-state O 3 (D 2 O) complexes. By measurement of the laboratory velocities and recoil angles of the O + O 2 + D 2 O products simultaneously, a kinematically complete description of the three-body dissociation dynamics of O 3 (D 2 O) is obtained. The results show that clustering of D 2 O to O 3 - stabilizes the system by 0.75 ( 0.09 eV. Photodetachment to the triplet states of O 3 in the complex results in three-body dissociation, with no clear evidence observed for quenching or intracluster reaction in the complex. The molecular-frame differential cross section (MF-DCS), showing how the three products scatter in the molecular frame, is presented and discussed in light of DFT calculations of possible equilibrium geometries of the parent O 3 - (D 2 O) anion. 1. Introduction The chemistry of tropospheric and stratospheric ozone has stimulated much research interest since the discovery of the impact of ozone in our atmosphere. There remain, however, puzzling issues to be resolved. Two such issues may involve three-body reaction dynamics. Locker et al. and Anderson, et al. suggested that the three-body recombination process O + O 2 + M f O 3 + M involving low-lying excited states of ozone may contribute to an understanding of the observed discrepan- cies between measured and calculated rates of ozone production in the atmosphere. 1,2 In addition, Mauersberger et al. recently proposed an explanation for isotopic fractionation of ozone in the stratosphere based on how the atomic and molecular oxygen collide in the three-body recombination reaction to form ozone, rather than the symmetry of the different isotopomers of ozone itself. 3 The experiments discussed in this paper study the three- body dissociation dynamics of the lowest excited states of O 3 clustered to D 2 O. While the present work does not directly probe the three-body recombination process on the ground-state sur- face, it provides the first insights into the three-body dynamics of the system. The experimental results for the O 3 (D 2 O) system will be compared with those for O 3 ; hence, a brief discussion of relevant O 3 chemistry is in order. Extensive studies have been conducted to understand the electronic spectrum of O 3 . At photon energies below 3 eV, the Chappuis and Wulf absorption bands of neutral O 3 have been assigned to several low-lying excited states, three of which are poorly characterized triplet states: 3 A 2 , 3 B 2 , and 3 B 1 . These states have been shown, experimentally and theoreti- cally, to lie slightly above the dissociation asymptote of O 3 to form the ground states of O + O 2 . 4-8 In a previous experiment in this laboratory, Garner, et al. implemented the photoelectron- photofragment coincidence (PPC) technique to demonstrate that the low-lying excited states of ozone promptly dissociate. 9 In these experiments, the dissociative photodetachment (DPD) of O 3 - that occurs when photodetachment produces an excited state of O 3 was studied by measuring the photoelectron and photo- fragment kinetic energy and angular distributions in coincidence. The results supported the suggestion that the dissociation proceeds via the antisymmetric stretch, leading to a distortion from C 2V to C s symmetry. 9 A more recent optical spectroscopy experiment conducted by Abel et al. shows lifetimes of 50- 100 ps for the lowest triplet excited state of O 3 . 10 In the present experiments, the three-body dissociation of O 3 (D 2 O) is studied by investigating DPD of the O 3 - (D 2 O) complex. Studies of three-body dissociation using the PPC technique are now possible with a new apparatus capable of multiparticle detection, enabling coincidence measurements of a photodetached electron and three or more neutral fragments. 11 This technique has been previously applied to the study of the three-body dissociation dynamics of O 6 . 12 When O 3 - (D 2 O) is photodetached at 258 nm, an electronically excited ozone-water complex may be formed and undergo the following DPD processes: In reaction 1, photodetachment of the anion precursor yields an excited O 3 (D 2 O) complex that undergoes dissociation to O + O 2 + D 2 O. This is the analogue of the process studied by Garner et al. in free O 3 . 9 The dissociation of excited O 3 is quenched in reaction 2, producing stable O 3 and D 2 O. Reaction 3 involves an intracluster reaction between the dissociating ozone and water moieties to form two hydroxyl radicals, which has been suggested to play a role in the formation of OH in the troposphere. 13,14 The PPC technique used in these experiments records the kinetic energy, mass, and angular distributions of an electron and three photofragments in coincidence. The complete kine- * To whom correspondence should be addressed. E-mail: rcontinetti@ ucsd.edu. O 3 - (D 2 O) + hν f O + O 2 + D 2 O + e - (1) f O 3 + D 2 O + e - (2) f 2OD + O 2 + e - (3) 10237 J. Phys. Chem. A 1999, 103, 10237-10243 10.1021/jp991975c CCC: $18.00 © 1999 American Chemical Society Published on Web 09/29/1999