We have determined the exchange splitting of the 1${\mathrm{\sigma }}_{\mathit{g}}^{\mathrm{-}1}$3${\mathrm{\sigma }}_{\mathit{u}}$ shape resonance excitation of gas phase ${\mathrm{O}}_2$, which cannot unambiguously be identified using conventional absorption spectroscopy. By identifying the electronic decay spectrum of the shape resonance excitation we could separately monitor the decay of the ${\mathrm{\sigma }}^{\mathrm{*}}$ shape resonance and the Rydberg states. Using this fingerprint method we measured the pure core to 3${\mathrm{\sigma }}_{\mathit{u}}$ bound state absorption spectrum in partial secondary electron yield spectroscopy.

• Received 15 July 1993

DOI:https://doi.org/10.1103/PhysRevLett.71.3091