The ionization potentials of NO2 up to 28 eV have been measured using He i and He ii high‐resolution photoelectron spectroscopy. The assignment of the ionized levels below 16 eV is straightforward, including the singlet–triplet splittings, and allows us to assign Rydberg series in the vacuum uv leading to the first five ionization potentials. Above 16 eV the spectrum is complicated by the absence of obvious singlet–triplet splittings. The assignments we propose fit in with a coherent scheme for the molecular‐orbital energy levels of 16‐, 17‐, and 18‐electron triatomic molecules. The SCF calculations have been performed on several states of NO2 and NO2+ (both open shell and closed shell) and the results are in quite good agreement with the experimental ionization potentials.

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