Surface core level shift of the Be 1s level observed from a Be(0001) single crystal surface

We have used surface sensitive photoelectron spectroscopy to study the Be 1s core level from the Be(0001) single crystal surface. Two core peaks are observed, one from the bulk atoms, and the other from the first layer surface atoms. They are separated in binding energy by 0.50 eV, the surface peak being located at lower binding energy. This experimental surface core level shift (SCS) is compared to calculated SCSs using different assumptions on the character of the initial and final state of the photoionized Be metal atoms in the bulk and at the surface. A final state having the Al (sp)³ valence configuration yields a SCS of the right direction and magnitude. The surface core peak is found to be broader than the bulk core peak. We attribute the main broadening in both the bulk and the surface peaks to phonon shake-up and discuss this using a simple model due to Flynn.