Penning ionization of thiophene, furan, and pyrrole by collision with He*(2³S) metastable atoms

Penning ionization of thiophene, furan, and pyrrole upon collision with He*(2³S) metastable atoms was studied by collision-energy-resolved Penning ionization electron spectroscopy. A strong negative collision energy dependence of the partial ionization cross sections was observed for π bands. This tendency was used to make an assignment for the lowest π orbital in the ultraviolet photoelectron spectrum. In the studied collision energy range, the interaction potentials between He*(2³S) and target molecules were found to be anisotropic, which is consistent with calculated interaction potential curves. For the out-of-plane directions, attractive interactions were found around the π orbital region. For the in-plane directions, repulsive interactions were found around the σ_CH orbital region and the sulfur atom of thiophene, while attractive interactions were found around the oxygen atom of furan. In the Penning ionization electron spectrum of pyrrole and thiophene, a satellite band was found to be associated with a shake-up ionization process from an occupied π orbital.