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

Abstract

The anion photoelectron imaging spectra of O·VOC and O·VOC (VOC = hexane, isoprene, benzene, and benzene-d) 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 O·VOC spectral features compared to the bare O indicate that photodetachment of the anion accesses a modestly repulsive region of the O-VOC potential energy surface, with subtle VOC dependence on the relative energies of the O (X Σ)·VOC ground state and O (a Δ)·VOC excited state. In contrast, a significantly higher intensity of the transition to the O (a Δ)·VOC excited state relative to the O (X Σ)·VOC ground state is observed for VOC = benzene, with a less pronounced effect observed for VOC = isoprene. Similar spectral effects are observed in the O·benzene and O·isoprene PE spectra. Several explanations are considered, with involvement of a temporary anion state emerging as the most plausible.

Photoelectrons Are Not Always Quite Free

“…Any vibrationally excited levels of the dipole-bound state that lie above the detachment continuum can autodetach via vibrational relaxation (Δ v = −1 propensity rule), resulting in an enhancement of the electron signal associated with that particular v ′ – 1 neutral level. A variation on this theme has also been observed in our own laboratory in the PE spectrum of the O 2 – ·benzene van der Waals complex . The enhancement of the broad PE signal peaked at electron kinetic energy, e – E K ≈ 1 eV, which coincides with the O 2 ( 1 Δ g )·benzene + e – ← O 2 – ( 2 Π g )·benzene + hv (3.49 eV) transition, which was attributed to resonance with a temporary anion state of benzene at e – E K = 1.12 eV .…”

“…It was not observed for homometallic Ce x O y − homologues, suggesting that the exceptionally high density of spin states in Sm centers is somehow involved in this phenomenon. Ce and Sm, both lanthanoids, differ in the 4f subshell occupancy in these clusters, with Ce having a singly occupied 4f subshell, and Sm having between 4f 5 and 4f 6 , depending on the specific Sm x Ce n−x O y / Sm x Ce n−x O y − composition (n = 2, 3, 1 ≤ x ≤ n, y ≤ n). 3 Excited anion states embedded in the detachment continuum can cause photon-energy-dependent intensity anomalies.…”

“…A variation on this theme has also been observed in our own laboratory in the PE spectrum of the O 2 − •benzene van der Waals complex. 5 The enhancement of the broad PE signal peaked at electron kinetic energy,…”

The electron shuffle: Cerium influences samarium 4f orbital occupancy in heteronuclear Ce–Sm oxide clusters

“…Threshold effects would have the opposite effect of what is observed. We have previously observed enhancements in photodetachment transitions associated with temporary anion states at e KE values that are resonant with the temporary anion state, but they have exhibited isotropic photoelectron angular distributions, 55 in contrast with what is observed in the Sm 2 O y (y = 1, 2) spectra.…”

Reply to “Comment on ‘Singlet Oxygen 1O2 in Photocatalysis on TiO2. Where Does It Come from?’”

“…This claim is not correct. The fact of the one-photon electron photodetachment from • O 2 – by radiation at 355 nm is well established in many studies carried out with the superoxide anion and its van der Waals complexes in molecular beams − (refs 35–40 in ref ). Probably the authors of the Comment mean that only a minor part of the pulse energy in experiments of ref could be used for the photodetachment.…”