Core excitation-induced photodesorption of molecular and fragment ions from CO adsorbates on metal surfaces

The photodesorption yields of CO⁺, O⁺, O²⁺, and C⁺ from CO adsorbed on several metal surfaces show marked increases with resonant structure around and above the C 1s and O 1s thresholds. While the molecular ions have low yields which follow essentially the core excitation cross sections (with some enhancement for multiple excitations), the predominant O⁺ fragment ion shows very strong increases considerably above O 1s, with superimposed resonant structure that must be assigned to threshold resonances of multiple excitations of the core-excited molecule (shake-up thresholds). These structures in the yield curves, which have been reported before, show very strong similarities for the various investigated CO species with quite different bonding (very weak bond on Cu (100); strong bonds on Ru (001) and on Ni (111), linear in the first and bridged in the second case; enhanced bond on K/Ni (111) with weakening of the intramolecular bond) which suggests that the relevant excitations are of intramolecular nature. On the other hand, the strong variations of desorption efficiencies with type of excitation and with substrate and coverage point to the importance of delocalization and quenching of the primary and/or secondary excitations. Polarization-dependent measurements have been used to determine the symmetries of the resonant transitions which together with their energies allow tentative assignments of the observed resonances; the overall polarization-independent increase is explained by shake-up and shake-off continua. Decay of all these complex primary excitations will lead to strongly correlated (and therefore highly localized), multiply valence excited states which can explain the high desorption efficiency compared to "normal" core excitations, as well as the selectivity observed. However, arguments are given that atomic motion starts during core life time so that ultrafast processes are partly responsible for the effects seen. This assumption leads to a more consistent picture.