Overlayer Core-level Shifts Induced by Limited Charge Transfer

Abstract

Photoemission studies of oxidized overlayer systems reveal shifts in overlayer core-levels to lower binding energies. Thin overlayers of Ce, ∼5–15 Å, were deposited on Al, Cu and Ta. About 1 monolayer K was deposited on Ta. The systems were subsequently exposed to oxygen. A common feature for these samples is the formation of a thin substrate oxide layer for relatively light doses of oxygen. The core-level shifts are explained by lack of Fermi-level equilibration between overlayer and substrate due to insufficient charge transfer. Core levels are therefore referenced to the local vacuum-level, and thus are sensitive to work-function changes.