Comparative study of photoionization and photofragmentation mechanisms of sulfur VOCs

YANINA B. BAVA; YANINA BERRUETA MARTÍNEZ; REINALDO L. CAVASSO FILHO; YENY A. TOBÓN; SOPHIE SOBANSKA; ROSANA M. ROMANO

Campinas

Otro; 26th RAU Annual Users Meeting LNLS; 2016

Laboratório Nacional de Luz Síncrotron

Photochemical studies of volatile organic compounds are relevant for the understanding of atmospheric chemistry. In this work, and as part of a general project dedicated to the elucidation of photofragmentation mechanisms generated from the interaction between the synchrotron radiation and compounds with atmospheric interest (see for example references 1 and 2, and references cited therein), we present the comparative study of photoionization and photofragmentation mechanisms of sulfur VOCs. We selected three model species: methyl thioglycolate (MTG), CH3OC(O)CH2SH, allyl methyl sulfide (AMS), CH2=CHCH2SCH3, and S-allyl thiopropionate (SATP), CH2=CHCH2SC(O)CH2CH3. These compounds were studied in the TGM and SGM beamlines at LNLS using synchrotron radiation in two energy regions, 7.3-300 eV and 100-1000 eV, respectively, using the chamber for gaseous samples and coincidence techniques. Total Ion Yield (TIY) spectra in the energy region corresponding to the S 2p, C 1s and O 1s core electrons were obtained. The TIY spectra around the S 2p threshold, ~173 eV, show two intense peaks corresponding to the spin-orbit coupling resonances, from S 2p1/2 and S 2p3/2 towards σ*(C?S). In the O 1s energy region only one peak was observed, corresponding to the transition from O 1s to π*(C?O), for the MTG and SATP molecules. Insted, the C 1s TIY spectra present different peaks assigned to resonances C1s à π*(C=O), C1s à π*(C=C) and C1s àσ*(C?S). ?Site specific excitation? was detected, allowing the identification of different kind of carbon atoms in each of the studied molecules. Photoelectron-Photoion-Coincidence (PEPICO) spectra were taken in the valence energy region at different irradiation energies starting from the ionization potential, while in the core electron energy regions the spectra were measured at each resonance and below and above every resonance. PE2PICO spectra were also obtained, and the fragmentation mechanisms were elucidated from the shape and the slope of the coincidence islands in these bidimensional spectra. In all the cases, HCS+ and H3+ (both interstellar ions) were detected as products of molecular ion fragmentation. The most abundant ions are CO+ (for MTG and SATP), C3H3+ (generated from the allyl groups), and HCS+ (for the three molecules). The PE2PICO spectra of MTG are dominated by the coincidence islands with the H+ ion and by other two islands, CH3+/HCS+ and COH+/HCS+. In the PE2PICO spectra of AMS the most important coincidence is C3H3+/HCS+ and for the SATP C2H3+/C3H3+. Three- and four-body dissociation channels were observed for each molecule and the comparison between them were proposed.