A surface core level shift (SCS) is a kind of core-level shift that often emerges in X-ray photoelectron spectroscopy spectra of surface atoms.
Reduced coordination leads to narrower valence bandwidth.
Such narrowing of the bandwidth increases the density of states, and if more than half of the valence band is filled, the band center is lower than bulk and the binding energy increases.
Because the binding energy in X-ray photoelectron spectroscopy is affected by the final state and other chemical environments, this simple explanation cannot always be applied to the interpretation of X-ray photoelectron spectra.
In spite of such complexity, the SCS gives important information about the chemical nature of surface atoms.