Total ion yield spectra and photofragmentation in S-Methyl Thioacetate, CH3C(O)SCH3, under VUV Irradiation

M. F. ERBEN; M. GERONÉS; R. M. ROMANO; C.O. DELLA VÉDOVA

Laboratório Nacional de Luz Síncrotron, Campinas, Brasil

Otro; 15ª Reunião Anual de Usuários (RAU); 2005

Laboratório Nacional de Luz Síncrotron

Recently we have started with the study of the inner shell electron structure and the dynamic of the ionic dissociation following photon absorption on the halogenocarbonylsulfenyl containing compounds (XC(O)SY). Particularly, the presence of a sulphur atom in the central group results in an invitation to perform measures in the S 2p ionisation edge. Thus, following these studies, we present here for the rst time the Total Ion Yield (TIY) spectra of CH3C(O)SCH3, in the 12-300 eV range (available at the TGM facility of the LNLS). This energy range allows us to record the S 2p and C 1s edges. Multicoincidence techniques, namely PhotoElectron-PhotoIon-Coincidence (PEPICO), and PhotoElectron-PhotoIon- PhotoIon-Coincidence (PEPIPICO) time of ight (TOF) mass spectrometry have been obtained below and above each ionisation edge. Furthermore, the C 1s and S 2p ionisation energies have been extracted from the TIY spectra and compared with those calculated based on the equivalent-core approximation taking OCS as a reference compound. The corresponding calculations were performed by using the Density Functional Theory (DFT) at the B3LYP level of approximation and the 6-311++G** basis sets, as implemented in the Gaussian 98 suite of programs. The TIY spectra below the S 2p edge is dominated by a group of three signals, where the two most intense could correspond to a spin-orbit split of the 2p term of the sulfur in 2p1/2 and 2p3/2, levels. Similar resonance features were observed for other members of the carbonylsulfenyl family of compounds already measured, like FC(O)SCl, ClC(O)SCl, CH3C(O)SH and CH3OC(O)SCl [1], [2], [3]. However, diferent patterns of resonance were found in each case, denoting a rather complex behaviour. The TOF mass spectrum of CH3C(O)SCH3 measured at a photon energy of 165.4 eV (at the S 2p resonance) shows important peaks corresponding to intense signals around 15 a.m.u./q. These are characteristic features for methyl group CHx+ (x= 0,1,2,3) being the signals at 43 a.m.u./q assigned to a CH3CO+ fragment. Surprisingly, the parent ion is present as an intense signal at 90 a.m.u./q. The analysis of the PEPICO spectra reveals interesting aspects related with the possibility of site-speci c fragmentation. Preliminary results show that important changes occur in the branching ratios of the ions with the incident photon energy. This results are rather interesting because in the previously studied XC(O)SY molecules FC(O)SCl and ClC(O)SCl, a lose memory efect or non speci c fragmentation after an Auger decay, promoted by the delocalization of the valence orbitals over the whole planar molecule, where invoked in order to explain the almost non-variation of the PEPICO spectra in the range of 100-1000 eV [1], [2]. Acknowledgements. This work was supported by LNLS under proposal D05A-TGM, Departamento de Química, Facultad de Ciencias Exactas, UNLP, CONICET, CIC, and Fundación Antorchas. References 1. Erben, M. F.; Romano, R. M.; Della Vedova, C. O., J. Phys. Chem. A. 108, 3938 (2004) 2. Erben, M. F.; Romano, R. M.; Della Vedova, C. O.; to be submited to J. Phys. Chem. A 3. Erben, M. F.; Romano, R. M.; Della Vedova, C. O.; 14 RAU, LNLS Campinas, 2004