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.