Adlayer Core-Level Shifts of Random Metal Overlayers on Transition-Metal Substrates

We calculate the difference of the ionization energies of a core electron of a surface alloy, i.e., a $B$ atom in a $A_{1-x}{B}_x$ overlayer on a fcc $B\left(001\right)$ substrate, and a core electron of the clean fcc $B\left(001\right)$ surface using density-functional theory. We analyze the initial-state contributions and the screening effects induced by the core hole, and study the influence of the alloy composition for a number of noble metal-transition metal systems. Our analysis clearly indicates the importance of final-state screening effects for the interpretation of measured core-level shifts. Calculated deviations from the initial-state trends are explained in terms of the change of inter- and intra-atomic screening upon alloying. A possible role of alloying on the chemical reactivity of metal surfaces is discussed.