Wigner Near-Threshold Effects 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 550 nm, resolving the vibrational and rotational structure of the NO₂(X²A₁) + e^- ← NO2-(X¹A₁) + 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 β ≈ 1.5 gradually increasing to β ≈ 1.7 upon approaching the threshold. However, at very low kinetic energies, within 0.1 eV of the threshold, β 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 β is reproduced by a new mixed spd orbital model, yielding a NO2-(X¹A₁) molecular-orbital decomposition of 2% p, 44% s, and 54% d character.