Physical Review B
The change in the average surface-atom core-level shift (SCS) produced by submonolayer coverages of alkali adsorbates is quite small for both Ta(110) and W(110), less than 45 meV in magnitude. The small change of the measured SCS’s as a function of coverage decisively supports the covalent-bonding picture of alkali adsorption proposed by Ishida and Terakura.
"Surface Core-Level Shifts of Ta(110) and W(110) vs. Alkali-Atom Coverage: Implications for the Alkali-Substrate Interaction," A. B. Andrews, D. M. Riffe, and G. K. Wertheim, Phys. Rev. B 49, 8396 (1994).