Photoelectron Imaging of OXIDE.VOC Clusters

Abstract

Perturbations of the bare O2-and O4-electronic structure arising from VOC (VOC = hexane, isoprene, benzene and benzene.D6) interactions are investigated using anion photoelectron imaging at 2.33 and 3.49 eV photon energies. Trends observed from comparing features in the spectra include VOC-identity-dependent electron affinities of the VOC complexes relative to the bare oxide clusters, due to enhance stability in the anion complex relative to the neutral. Autodetachment is observed in all O4-.VOC spectra and only isoprene with O2-. In addition, the intensities of transitions to states correlated with the singlet states of O2 neutral via detachment from the O2-.VOC anion complexes show dramatic VOC-identity variations. Most notably, benzene as a complex partner significantly enhances these transitions relative to O2-and O2-.hexane. A less significant enhancement is also observed in the O2-.isoprene complex. This enhancement may be due to the presence of low-lying triplet states in the complex partners.

O2–·[Polar VOC] Complexes: H-Bonding versus Charge–Dipole Interactions, and the Noninnocence of Formaldehyde

“…In contrast to our previously reported spectra on complexes formed between O 2 − and nonpolar VOC's, 16 band a in the O 2 −…”

“…30,31 Oxygenated VOCs such as 2-propenol (allyl alcohol), 3-buten-1-ol, and 3-buten-2-ol are largely anthropogenic 32,33 and lead to secondary aerosol formation. 34 A significant difference between the O 2 − •[polar VOC] complexes in the current study and the O 2 − •[nonpolar VOC] complexes studied previously 16 arises from the strong charge− dipole interactions and hydrogen bonding that favor a distinct structural orientation between the O 2 − anion and the neutral VOC. In the case of nonpolar partners, the intermolecular potential energy surface was inferred to be flat, on the basis of the numerous close-lying structural minima separated by low barriers found computationally.…”

“…We also consider an alternative explanation for the unique profile of the O 2 − •formaldehyde spectrum. In our previous study on O 2 − •[nonpolar VOC] complexes, 16 we reported an anomalous feature in the spectrum of O 2 − •benzene, the apparent dramatic enhancement of the transition to the a 1 Δ g •[S 0 ] neutral state, with a more isotropic PAD. The e − KE at which this enhancement was observed coincided with the e − KE of a similar intensity anomaly in the O 4…”

“…Our results showed that the O 2 (a 1 Δ g )•VOC lifetimes and energy relative to the O 2 (X 3 Σ g − )•VOC ground electronic state varied with VOC identity. The work presented herein is an extension of the previous study 16 to polar VOCs containing an alcohol or a carbonyl substituent group.…”

“…11−15 We recently reported the results of a study of neutral O 2 •VOC (VOC = hexane, isoprene, benzene, and benzene-d 6 ) collision complexes by photoelectron imaging (PEI) spectroscopy of the anionic precursor. 16 Photodetachment of the anionic complex prepares the neutrals on a repulsive region of the O 2 −VOC intermolecular potential energy surface. Our results showed that the O 2 (a 1 Δ g )•VOC lifetimes and energy relative to the O 2 (X 3 Σ g − )•VOC ground electronic state varied with VOC identity.…”