Photoelectron Spectroscopy of SO 3 - at 355 and 266 nm S. Dobrin, B. H. Boo, ² L. S. Alconcel, and R. E. Continetti* Department of Chemistry and Biochemistry, UniVersity of California, San Diego, 9500 Gilman DriVe, La Jolla, California 92093-0314 ReceiVed: July 19, 2000; In Final Form: September 11, 2000 Photoelectron spectra of SO 3 - were recorded at 266 and 355 nm to study photodetachment of the SO 3 - anion ( 2 A 1 ) to the ground state of neutral SO 3 ( 1 A′ 1 ). A long vibrational progression in the 355 nm spectrum is attributed to excitation of the umbrella mode, ν 2 , consistent with predictions that C 3V symmetry SO 3 - yields D 3h symmetry SO 3 upon photodetachment. At 266 nm, photodissociation of SO 3 - to SO 2 + O - was also observed. The geometry and normal-mode frequencies of SO 3 - and SO 3 as well as the adiabatic electron affinity (AEA) and vertical detachment energy (VDE) of SO 3 have also been calculated with ab initio (MP2 and CCSD(T)) and DFT methods. Using theoretical predictions and experimental data, Franck-Condon simulations of the photoelectron spectra were found to be in good agreement with experiment. The calculated AEA agreed well with experiment, but the VDE was found to be less accurate, presumably because of the large geometry change between anion and neutral. 1. Introduction The structure and energetics of the sulfur oxides are of interest due to the important role played by these species in a variety of environmental problems. Among them, sulfur trioxide (SO 3 ) has attracted attention as an intermediate in the conversion of SO 2 to H 2 SO 4 , forming acid rain. 1 The kinetics and mechanism of this reaction have been intensively studied experimentally 2-4 and theoretically. 5-8 Despite the reactivity of SO 3 , a number of studies have shown that it is a planar molecule with D 3h symmetry in the ground state. These studies have included infrared, Raman and photoionization studies of gas-phase SO 3 9-13 and laser flash photolysis and infrared studies in matrix isolation experiments. 14,15 In the matrix isolation experiments, several isomers of SO 3 have also been identified. 15 These experimental studies have been complemented by a number of ab initio calculations on the ground electronic state of SO 3 . 7,8 The most recent of these include a detailed study of the thermochemistry and calculation of a spectroscopic quality force field for SO 3 using the CCSD(T) method and large basis sets including inner-shell correlation by Martin. 16,17 The SO 3 - anion, however, has received considerably less attention. Experimentally, the UV absorption spectrum of the anion was measured by pulsed radiolysis 18 and matrix-isolation FTIR studies have been carried out. 19 The electron affinity of SO 3 has been measured by collisional ionization 20 and in electron-transfer experiments between SO 3 - and various colli- sion partners. 21 An unresolved portion of the SO 3 - photoelectron spectrum at 355 nm was recently reported by Ding, et al. in a study of the doubly charged S 2 O 8 2- anions. 22 Theoretical studies of the SO 3 - anion to date have been carried out at relatively low levels using small basis sets and approximate methods. Margrave and co-workers carried out UHF SCF/3-21+G* calculations to aid in the interpretation of matrix-isolation FTIR spectra. 19 More recently, in a study of mono- and di-sulfoxy anions, McKee performed DFT/6-31+G(d) and post-Hartree- Fock (MP2, G2, QCISD(T)) calculations with the same basis set, yielding values for the adiabatic electron affinity of SO 3 ranging from 2.15 to 2.50 eV. 23 These calculations also show the anion to be pyramidal with C 3V symmetry. In the present study, photoelectron spectra of SO 3 - have been measured at 355 and 266 nm. It is found in the photoelectron spectra that photodetachment of pyramidal SO 3 - produces SO 3 far from the equilibrium geometry, leading to significant excitation of the out-of-plane bending motion (i.e., ν 2 or “umbrella” mode). As demonstrated by Ellison et al. in a study of CH 3 and CH 3 - , 24 photoelectron spectroscopy on a system like this provides insights into the equilibrium geometry of the anion and the potential curve for the ν 2 mode in the neutral molecule. In this paper, the measured vertical detachment energy of SO 3 is compared with the results of DFT and ab initio calculations. Franck-Condon factors for the ν 1 and ν 2 modes have been calculated and used to analyze the experimentally observed vibrational structure. 2. Experimental Details The photoelectron kinetic energy spectra of SO 3 - at 355 and 266 nm were measured with a fast-beam photoelectron spec- trometer. The apparatus has been described in detail previ- ously. 25,26 Briefly, a mixture of Ar, N 2 O and SO 2 expands into vacuum through a pulsed valve and subsequently passes through a pulsed electrical discharge 27 where negative ions are produced. The ions are cooled in a supersonic expansion, accelerated to 3 keV, mass-selected by time-of-flight and collimated before interaction with a polarized Nd:YAG laser beam (266 or 355 nm, 100 ps pulse width, ≈15 mJ/cm 2 fluence, 1 kHz repetition rate). Photoelectrons are detected in the direction perpendicular to the plane of the laser and ion beams by a large, solid-angle, time- and position-sensitive detector. 25 Using the time-of-flight through a field-free region and the position of detection, the recoil angle of each photoelectron was calculated, allowing correction of the electron laboratory kinetic energy to the center- of-mass electron kinetic energy (eKE). 28 The present experi- * Corresponding author. E-mail rcontinetti@ucsd.edu. ² Permanent address: Department of Chemistry, Chungnam National University, Taejon 305-764, Korea and Center for Molecular Science, 373-1 Kusung-dong, Yusung-gu, Taejon 305-701, Korea. 10695 J. Phys. Chem. A 2000, 104, 10695-10700 10.1021/jp0025680 CCC: $19.00 © 2000 American Chemical Society Published on Web 10/21/2000