Wigner Near-Threshold Efects in the Photoelectron Angular Distribution of NO−2

High-resolution velocity-map imaged photoelectron spectra of the nitrite anion NO2- are measured over a range of photodetachment wavelengths between 355 and 550nm, resolving vibrational and rotational structure of the NO2(X2A1)+e- ← NO2-(X1A1)+hν transition. A full rotational band model is constructed to define 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 β ∼ 1.5 gradually increasing to β ∽ 1.7 on the approach to threshold. However, at very low kinetic energies, within 0.1eV of threshold, β dramatically drops to 0. This behaviour 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 threshold is favoured. The full kinetic-energy dependence of β is reproduced by a new mixed s-p-d orbital model, yielding an NO2-(X1A1) molecular-orbital decom...