X-ray-absorption spectrum of O2+

The x-ray absorption spectrum of the O2+ molecular cation is measured. The ions are stored in a cryogenically cooled radiofrequency ion trap and probed by tunable synchrotron radiation. The spectrum exhibits several salient features: a three-state composite π∗ resonance at the low-energy side followed by a two-component exchange split, a highly dissociative, σ∗ resonance pulled down well below the ionization limit; and a complex valence-Rydberg high-energy part, including several resolved bands. Small structures are interpreted as correlation state satellites with leading internal or semi-internal configurations. Calculations using the restricted active space wave functions and quantum wave packet dynamics offer an overall excellent interpretation of the spectral features.