Photoelectron Imaging Spectra of O2 –·VOC and O4 –·VOC Complexes

The anion photoelectron imaging spectra of O2 –·VOC and O4 –·VOC (VOC = hexane, isoprene, benzene, and benzene-d 6) complexes measured using 3.49 eV photon energy, along with the results of ab initio and density functional theory results are reported and analyzed. Photodetachment of these anionic complexes accesses neutrals that model collision complexes, offering a probe of the effects of symmetry-breaking collision events on the electronic structure of normally transparent neutral molecules. The energies of O2 –·VOC spectral features compared to the bare O2 – indicate that photodetachment of the anion accesses a modestly repulsive region of the O2–VOC potential energy surface, with subtle VOC dependence on the relative energies of the O2 (X 3Σg –)·VOC ground state and O2 (a 1Δg)·VOC excited state. In contrast, a significantly higher intensity of the transition to the O2 (a 1Δg)·VOC excited state relative to the O2 (X 3Σg –)·VOC ground state is observed for VOC = benzene, with a less pronounced effect observed for VOC = isoprene. Similar spectral effects are observed in the O4 –·benzene and O4 –·isoprene PE spectra. Several explanations are considered, with involvement of a temporary anion state emerging as the most plausible.