Origin of metallic surface core-level shifts

The unique property of the open 4f energy shell in the lanthanide metals is used to show that the initial-state energy shift gives an insufficient description of surface core-level shifts. Instead a treatment, which fully includes the final-state screening, account for the experimentally observed surface shifts of the occupied as well as the unoccupied 4f states. The surface energy shift of the initial state corresponds approximately to the average of the shifts for the occupied and unoccupied 4f levels.