Evidence for multi-electron excitations in photon stimulated ion desorption: CO and NO on Ni(100)

Abstract

The photon stimulated ion desorption (PSID) from saturation layers ( T ≲ 150 K) of CO and NO on Ni(100) has been monitored through the C(1s), N(1s), O(1s) and Ni(2p) core ionization thresholds. O + ions are found to be the major desorption product of both molecular adsorption systems. Whereas the O + yield from CO exhibits an absorption edge jump at the C(1s) threshold, no jump is found at the N(1s) threshold in the case of NO. Above the O(1s) threshold, the O + yield is found to deviate significantly from the O(KVV) Auger yield for both molecules. It exhibits an onset delayed by more than 20 eV and a broad maximum more than 100 eV above threshold. This indicates that, above the O(1s) threshold, the O + yield is dominated by desorption processes which are assisted by multi-electron excitations. Fine structure in the O + yield from both molecules around 50 eV above threshold is assigned to a 1 σ, 3 σ two-electron excitation where one electron is excited into the σ shape resonance of the molecules.