Photoionization of thiophene and 2-methylthiophene in the S 2p edge: influence of the methyl radical substitution in the fragmentation channels*

Thiophene (C4H4S) is the simplest aromatic structure bearing sulfur, derived from benzene by the replacement of two CH groups by sulfur. Previous synchrotron-based studies probed thiophene photoionization by C 1s and S 2p photoelectron spectroscopy (Giertz et al.,2002), electron resolved PEPIPICO in the S 2p edge (Kukk et al., 2015) and core-hole clock spectroscopy in the S K-edge (Martins, 2021), whereas the molecular dynamics were investigated by FEL (Kukk, 2021). Methylthiophenes are formed by the replacement of a hydrogen atom in the thiophene by the methyl group. In contrast to the highly studied thiophene, methylthiophenes lack experimental data on their photoionization processes, with most of the spectroscopic studies on them focusing on their polymers (Rocco, 2004; Santa Rita, 2011). Thiophene, 2-methylthiophene, 3-methylthiophene and presumably benzothiophene were detected on Martian soil (Eigenbrode et al., 2018) through EGA and CG-MS onboard the Curiosity rover, along with 3-methylthiophene and presumably benzothiophene. Their discovery raised the question of its origin, whether biotic or abiotic (Heinz, 2020).

We have compared the stability and the photofragmentation pattern of thiophene and 2-methylthiophene upon photoionization. Photons from the Pleiades beamline at the French synchrotron facility SOLEIL ionized the samples above the S 2p edge, at 171 eV. Cationic species resulting from the photoionization process were analyzed according to the ejected electrons binding energy. NEXAFS measurements of both molecules in the S 2p edge were obtained and will be compared to theoretical results. 

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J. B. Martins, Thèse de Doctorat, Sorbonne Université, 2021.

E. Kukk et al., Phys. Rev. Research, 3, 013221, 2021.

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