REFERENCING CORE LEVELS IN PHOTOELECTRON-SPECTROSCOPY

Photoemission studies of oxidized alkali-metal overlayer systems reveal that the alkali-metal core levels may be referenced to the vacuum level instead of the Fermi level. Shifts in alkali-metal core levels are observed to track changes in the system work function. This phenomenon is caused by an inability to transfer charge to equilibrate Fermi levels, on the time scale of the photoemission process, and has been observed in O2/K/Ta and O2/K/Al systems. Upon exposure to oxygen, one monolayer of oxidized potassium forms on top of a thin substrate oxide layer. The K core-level binding energies are observed to be referenced to the vacuum level for substrate oxide thicknesses down to about 5. © 1990 The American Physical Society.