Dissociative Photodetachment Dynamics of S 2 O 2 - Todd G. Clements, Hans-Ju 1 rgen Deyerl, ² and Robert E. Continetti* Department of Chemistry and Biochemistry, UniVersity of California, San Diego, 9500 Gilman DriVe, La Jolla, California 92093-0340 ReceiVed: August 28, 2001; In Final Form: NoVember 5, 2001 Photoelectron-photofragment coincidence spectroscopy is employed to study the dissociative photodetachment (DPD) dynamics of S 2 O 2 - at 258 nm. Experimental data and theoretical calculations show evidence for photodetachment from a trigonal form of S 2 O 2 - . The vertical detachment energy of this isomer was determined to be 3.73 ( 0.02 eV. An upper bound of 2.41 ( 0.14 eV is determined for the enthalpy of the reaction S 2 O 2 - f S + SO 2 + e - at 0 K. The observed dynamics are interpreted in terms of dissociative photodetachment of S 2 O 2 - to S( 3 P) + SO 2 ( 1 A 1 ) + e - , S( 1 D) + SO 2 ( 1 A 1 ) + e - , and S 2 ( 3 Σ g - ) + O 2 ( 3 Σ g - ) + e - product channels. The S-atom channels are characterized by a large photofragment kinetic energy release and an anisotropic photofragment angular distribution peaked along the electric vector of the laser. The S 2 channel has a low kinetic energy release consistent with elimination of highly vibrationally excited O 2 from a strained form of the trigonal isomer. 1. Introduction Sulfur monoxide (SO) acts as an intermediate in the oxidation of sulfur compounds in combustion 1 as well as the atmosphere. 2 It may also be an important participant in the atmospheric chemistries of Venus 3 and Io. 4 As a result, the energetics and reaction dynamics of this molecule are of considerable interest. Herron and Huie studied the kinetics of formation of the transient SO dimer (S 2 O 2 ), created by the reaction SO + SO + M f (SO) 2 + M and concluded that the (SO) 2 cluster is relatively long-lived. 5 Harcourt proposed that a triplet S 2 O 2 intermediate plays a role in the formation of SO 2 and S 2 O from SO. 6 The stability and dissociation dynamics of these intermedi- ates can have a significant effect on interpretations of the chemical kinetics of SO, 5 but experimental information on these clusters is scarce. Photoelectron photofragment coincidence (PPC) spectroscopy has been shown to be a valuable method for studying the energetics and dissociation dynamics of transient neutral molecules after photodetachment from the anion. 7,8 In this paper, PPC spectroscopy is applied to S 2 O 2 - , and represents the first investigation involving the anionic form of this molecule. There have been a number of previous spectroscopic studies of neutral S 2 O 2 . Lovas et al. observed S 2 O 2 in a microwave discharge of SO 2 . 9 In those experiments, S 2 O 2 was found to have a cis-planar OSSO structure similar to that in Figure 1a. A number of theoretical studies have been performed on S 2 O 2 10-13 and although only the cis-planar structure has been observed experimentally, Marsden and Smith calculated thirteen singlet and six triplet isomers of S 2 O 2 . 10 They concluded that a trigonal planar form of S 2 O 2 (Figure 1b), similar in structure to SO 3 is more stable than the cis-planar isomer, but was not observed in the microwave discharge spectra due to the relatively high energies of molecular products from that method. The photoionization efficiency spectrum of S 2 O 2 has also been measured in the range 650-1250 nm 14 and 105-130 nm, 15 with the latter study yielding an adiabatic ionization energy of 9.93 ( 0.02 eV. We present the photoelectron spectrum, the energy-dependent photofragment angular distribution, and the photoelectron photofragment correlation spectrum of the anion S 2 O 2 - . From these data we show evidence for the trigonal isomer of S 2 O 2 - (Figure 1d) and determine the energetics and dynamics of the dissociation of the corresponding neutral (Figure 1b). 2. Experimental Section The photoelectron photofragment coincidence spectrometer has been described before in detail 16 and will be reviewed only briefly here. The source gas configuration differs from previous descriptions of this technique. Here, S 2 O 2 - is produced by electron impact on two crossed continuous gas expansions, similar to (although simpler than) entrainment methods used by Johnson and co-workers 17 and discussed in early supersonic- ² Current address. Research Center COM, DTU, DK-2800 Kgs. Lyngby, Denmark. * Corresponding author. E-mail: rcontinetti@ucsd.edu. Figure 1. Calculated geometries of S2O2 at the B3LYP/6-311+G(d) level. (a) cis-planar S2O2, (b) trigonal planar S2O2, (c) cis-OSSO - (the SO bond is rotated by 40.3° out of plane), and (d) trigonal S2O2 - . Distances are given in Å, and bond angles in degrees. 279 J. Phys. Chem. A 2002, 106, 279-284 10.1021/jp013329v CCC: $22.00 © 2002 American Chemical Society Published on Web 12/20/2001