Probing neutral atmospheric collision complexes with anion photoelectron imaging.*

Photodetachment of anionic precursors of neutral collision complexes offers a way to probe the effects of symmetry-breaking collision events on the electronic structure of normally transparent molecules. We have measured the anion photoelectron imaging (PEI) spectra of a series of O$_{\mathrm{2}}^{\mathrm{-}}\cdot X$ complexes, where $X$ is a volatile organic molecule with atmospheric relevance, to determine how the electronic properties of various $X$ molecules affect the low-lying electronic structure of neutral O$_{\mathrm{2}}$ undergoing O$_{\mathrm{2}}-X$ collisons. The study was motivated by the catalog of vibrational and electronic absorption lines induced by O$_{\mathrm{2}}-$O$_{\mathrm{2}}$, O$_{\mathrm{2}}-$N$_{\mathrm{2}}$, and other collisions. The energies of electronic features observed in the anion PEI spectra of O$_{\mathrm{2}}^{\mathrm{-}}\cdot X$ ($X \quad =$ hexane, hexene, isoprene and benzene) relative to O$_{\mathrm{2}}^{\mathrm{-}}$ PEI spectroscopic features indicate that photodetachment of the anion does indeed access a repulsive part of the O$_{\mathrm{2}}$ -- $X$ potential. In addition, the spectra of the various complexes show an interesting variation in the intensities of transitions to the excited O$_{\mathrm{2}}(^{\mathrm{1}}\Delta_{\mathrm{g}})\cdot X$ and O$_{\mathrm{2}}(^{\mathrm{1}}\Sigma _{\mathrm{g}}^{\mathrm{+}})\cdot X$ states relative to the ground O$_{\mathrm{2}}(^{\mathrm{3}}\Sigma _{\mathrm{g}}^{\mathrm{-}})\cdot X$ state. With $X \quad =$ non-polar species such as hexane, the relative intensities of transitions to the triplet and singlet states of O$_{\mathrm{2}}\cdot X$ are very similar to those of isolated O$_{\mathrm{2}}$, while the relative intensity of the singlet band decreases and becomes lower in energy relative to the triplet band for $X \quad =$ polar molecules. A significant enhancement in the intensities of the singlet bands is observed for complexes with $X \quad =$ isoprene and benzene, both of which have low-lying triplet states. The role of the triplet states in isoprene and benzene, and the implications for induced electronic absorption in O$_{\mathrm{2}}$ undergoing collisions with these molecules, are explored.