Wigner Near-Threshold Effects in the Photoelectron Angular Distribution of NO2.

High-resolution velocity map imaged photoelectron spectra of the nitrite anion NO2- are measured over a range of photodetachment wavelengths between 355 and 550 nm, resolving the vibrational and rotational structure of the NO2(X̃2A1) + e- ← NO2-(X̃1A1) + hν transition. A full rotational band model is constructed to define the spectroscopic constants of both the neutral and the lesser studied anion ground states. The corresponding photoelectron angular distributions are characterized by a large positive anisotropy parameter, with ß ≈ 1.5 gradually increasing to ß ≈ 1.7 upon approaching the threshold. However, at very low kinetic energies, within 0.1 eV of the threshold, ß dramatically drops to 0. This behavior is a consequence of the Wigner near-threshold selectivity of the electron partial-wave cross sections, whereby an atomic p-like orbital character adjacent to the threshold is favored. The full kinetic energy dependence of ß is reproduced by a new mixed spd orbital model, yielding a NO2-(X̃1A1) molecular-orbital decomposition of 2% p, 44% s, and 54% d character.

The dicarbon bonding puzzle viewed with photoelectron imaging.

Bonding in the ground state of C[Formula: see text] is still a matter of controversy, as reasonable arguments may be made for a dicarbon bond order of [Formula: see text], [Formula: see text], or [Formula: see text]. Here we report on photoelectron spectra of the C[Formula: see text] anion, measured at a range of wavelengths using a high-resolution photoelectron imaging spectrometer, which reveal both the ground [Formula: see text] and first-excited [Formula: see text] electronic states. These measurements yield electron angular anisotropies that identify the character of two orbitals: the diffuse detachment orbital of the anion and the highest occupied molecular orbital of the neutral. This work indicates that electron detachment occurs from predominantly [Formula: see text]-like ([Formula: see text]) and [Formula: see text]-like ([Formula: see text]) orbitals, respectively, which is inconsistent with the predictions required for the high bond-order models of strongly [Formula: see text]-mixed orbitals. This result suggests that the dominant contribution to the dicarbon bonding involves a double-bonded configuration, with 2[Formula: see text] bonds and no accompanying [Formula: see text] bond.

NOO Peroxy Isomer Exposed with Velocity-Map Imaging.

The chemistry of NO2, a key atmospheric trace gas, has historically been interpreted in terms of the C2v isomer ONO, with the peroxy isomer NOO only postulated to be stable. In this work, a velocity-map-imaged photoelectron spectrum of the nitrite anion, NO2-, reveals energetic-electron structure that may only occur by photodetachment from the NOO-(X̃1A') isomer. This measurement defines NOO(X̃2A') bond frequencies and an electron affinity of only 335(30) cm-1, which, supported by ab initio calculations, confirm the first observation of this important reactive species.