Chemical Physics 166 (1992) 229-239 North-Holland A study of the S2 (X 3x; ) molecule by multiphoton ionization spectroscopy M. Barnes ‘, J. Baker ‘, J.M. Dyke and R. Richter Department of Chemistry, The University, Southampton SO9 SNH, UK Received 6 April 1992 In this work the S2 molecule has been studied using resonance enhanced multiphoton ionization (MPI) spectroscopy in the single-photon wavelength region 285-320 nm. This has resulted in the first spectroscopic observation of a gerade Rydberg state of Sz. The observed bands, which show spin-orbit structure, have been assigned to a two-photon resonant process, ‘Ag( 3dx) +-X %g. Transitions to three vibrational levels (v’=O, 1, 2) of the excited state have been recorded. This has enabled determination of vibrational constants (tic and u&x, ) and the spin-orbit coupling constant for the ‘$ state, whilst rotational constants (B,. ) were derived from band head separations. A band in the MPI spectrum of Sz corresponding to the one-photon resonance B ‘Z; , v’= 8+X ‘Z; , v”=O has also been observed. This has been detected in both the S: and S+ mass channels. Possible mechanisms for production of these ions via the B ‘Z; state are discussed. 1. Introduction In this work (2 + 1) resonance enhanced multi- photon ionization (REMPI) spectroscopy has been used to study a ‘4 Rydberg state of SZ. This repre- sents the first MPI investigation of Sz and the first spectroscopic identification of a Rydberg state of SZ with gerade symmetry. In contrast, 02, which is va- lence isoelectronic with SZ, has been the subject of a number of REMPI studies, and a number of one- photon forbidden, two-photon allowed g-g transi- tions from the X ‘Z:g and a ‘ABO2 states to Rydberg states of gerade symmetry have been studied using both REMPI ion and photoelectron spectroscopy [ l- zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFE 91. The S2 molecule has been observed in flames, shock tubes and discharges containing sulphur compounds [ 10,111 and several S2 electronic transitions have been proposed as possible candidates for a chemical Correspondence to: J.M. Dyke, Department of Chemistry, The University, Southampton, SO9 5NH, UK ’ Present address: Department of Chemistry, The University of British Columbia, Vancouver, BC, Canada V6T 1Zl. 2 Present address: Department of Chemistry, Queen Mary and Westfield College, University of London, Mile End Road, Lon- don, UK. laser [ 12,131. Its first ionization energy, correspond- ing to the process S$ X zIIp,,,Z t-$X ‘Z;, has been measured as 9.356kO.002 and 9.38 +O.Ol eV by photoionization mass spectrometry [ 14 ] and photo- electron spectroscopy [ 15 ] respectively. Previous spectroscopic investigations on S, (X 3Zg ) have con- centrated on characterizing valence states and some low-lying Rydberg states of ungerade symmetry in one-photon transitions. Although the 3C; ground state has been the subject of infrared emission, EPR and microwave studies [ 16-l 81, the most exten- sively studied state of S2 is the B 3Z; valence state which has been observed both by one-photon absorp- tion from, and emission to, the X 3X; ground state [ 18-241. The B 3Z; state is heavily perturbed by the B” 311Ustate. It has a radiative lifetime of x 40 ns, whilst the B” 311U state has a lifetime of zyxwvutsrqponmlk 1 zyxwvutsrqponml OO- 1000 ns and this difference enabled the weaker B”+X emis- sion to be observed, after the stronger B-+X emission had decreased to zero, using “lifetime separated” laser-induced fluorescence [ 25 1. A valence state of gerade symmetry, the B’ ‘&state, has also been ob- served by studying its emission spectra to the A’ 3AU and A ‘Zz states. This emission was produced from a microwave discharge of a sulphur/helium mixture 1261. 0301-0104/92/$05.00 0 1992 Elsevier Science Publishers B.V. All rights reserved,