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 C_2v 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.