Origin of contrasting surface core-level shifts at the and surfaces

As expected, very similar oscillatory multilayer relaxations exist at the (Formula presented) surfaces of the isoelectronic Be and Mg. But surprisingly, these two systems exhibit drastically different behaviors in the surface core-level shift (SCLS). For Be, our density-functional theory calculations show that the SCLS persists down to the fifth layer, and depends strongly on lattice relaxation and final-state effect. In contrast, for Mg only the atoms in the first layer have a small and insensitive SCLS. The physical origin of the disparate features in the SCLS can be traced to the different bonding natures of the two elements in bulk form: strongly covalentlike for Be, and nearly free-electron-like for Mg.