Autoionization versus photoionization of molecular adsorbates:CO2physisorbed on Ni(110)

We compare the cross sections for valence-electron photoionization and autoionization after C 1s–to–bound and O 1s–to–bound excitation of ${\mathrm{CO}}_2$ physisorbed on Ni(110). Clear evidence is found for different selection rules for radiative excitation versus radiationless decay into the same ion states. All ion states that belong to the class of two-hole–one-particle states, in particular those in the inner-valence-electron regime, where the one-particle picture for photoionization is known to break down, are observed with very large intensities in autoionization spectra, but are hardly observable at all in photoionization. Combining photoionization and autoionization, we can characterize the ion-state wave functions according to their oxygen and carbon parentage. Differences between O 1s→2${\mathrm{\pi }}_{\mathit{u}}$ and C 1s→2${\mathrm{\pi }}_{\mathit{u}}$ decay into the normal single-hole states are discussed in terms of symmetry breaking via electron-hole localization and vibrational coupling in the core-to-bound excited state. Our interpretation is based on an ab initio Green’s-function calculation for the ion states in the outer and inner valence regions followed by explicit calculations of the radiationless autoionization rates.