Cs5p and 4d photoelectron linewidths of Cs/Ru(001): Implication for Cs-Ru interaction at the interface

Photoelectron linewidths of Cs5p and 4d levels of Cs adsorbed on Ru(001) for a series of coverages varying from submonolayer to multilayer have been examined. It is found that linewidth broadening for Cs atoms in direct contact with Ru occurs for both levels when compared with that of bulk Cs atoms, and that while Cs4d levels broaden slightly but persistently over the entire range of coverage, the 5p linewidth exhibits a sharp change between Cs atoms that are in direct contact with the Ru substrate and those that are not; that is at submonolayer, monolayer and thin multilayer coverage, the Cs5p peak of the Cs atoms which are in direct contact with the Ru substrate broadens considerably in comparison with the Cs5p peak of the Cs atoms in the second overlayer and in Cs metal. The Cs5p linewidth broadening is attributed primarily to the relatively short lifetime of the core hole resulting from a fast Coster-Kronig/super Coster-Kronig decay channel facilitated by Cs-Ru interaction at the interface. This channel, which is absent in the primary decay of the Cs4d core hole, has little effect on the 4d linewidth. This Cs5p linewidth broadening appears to be general for Cs in direct contact with any metal substrate and is discussed in terms of a s-d charge redistribution (rehybridization) model with the assistance of relativistic Dirac-Fock calculations. The less dramatic Cs4d broadening is attributed to a combined effect of interfacial phonon, chemical inhomogeneity and crystal filed broadening.