Optical luminescence excitation spectra of molecular oxygen in the soft x-ray region

The observation of an anomaly in the optical luminescence excitation spectrum of oxygen in the region of the oxygen K edge is reported. Dispersed luminescence spectra were obtained for x-ray excitation at the pi and sigma resonances, at the anomaly, and in the continuum. These spectra indicate enhanced production of O2+2 ions at both the sigma resonance and at the anomaly. The anomaly thus is attributed to a shake-up or shake-off state associated with an antibonding sigma molecular orbital of oxygen. This work also demonstrates that optical luminesence spectra provide state-specific information about the products of core hole excitation and relaxation.