Structure determination of heteroaromatic thiophene dimers by intermolecular Coulombic decay upon electron-impact ionization

This paper reports the structure and intermolecular potential energy curve (PEC) of heteroaromatic thiophene dimers obtained using a cold-target recoil-ion momentum spectroscopy reaction microscope. The three-dimensional momenta and kinetic energy release (KER) of the ${\mathrm{C}}_{4}{\mathrm{H}}_{4}{\mathrm{S}}^{+}+{\mathrm{C}}_{4}{\mathrm{H}}_{4}{\mathrm{S}}^{+}$ ion pairs are obtained by coincident momentum measurement. The two-body dissociation channel is initiated either by the rapid removal of one outer-valence electron from each thiophene molecule via sequential ionization or by intermolecular Coulombic decay (ICD). Our analysis of the absolute cross sections indicates that the ${\mathrm{C}}_{4}{\mathrm{H}}_{4}{\mathrm{S}}^{+}+{\mathrm{C}}_{4}{\mathrm{H}}_{4}{\mathrm{S}}^{+}$ ion pair is preferentially formed by the ICD process. The measured KER and PEC, accompanied by ab initio molecular dynamics simulations for three different types of conformers, enable us to clarify the minimum energy configuration of thiophene dimers in the gas phase. These results are helpful for our understanding of $\ensuremath{\pi}\text{\ensuremath{-}}\ensuremath{\pi}$ and $\ensuremath{\sigma}\text{\ensuremath{-}}\ensuremath{\pi}$ interactions within heteroaromatic ring complexes.