COVERAGE-DEPENDENT CORE-LEVEL PHOTOEMISSION INVESTIGATIONS OF NA/CU(111) AND NA/NI(111)

Coverage-dependent core level binding energies of Na adsorbed on both Cu(111) and Ni(111) surfaces have been measured by angle-integrated photoemission spectroscopy. For increasing Na coverage, the core level binding energies of the Na 2p and 2s levels decreased by 0.93 and 0.81 eV for Cu(111), and 1.1 and 0.95 eV for Ni(111), respectively, up to completion of the first monolayer. The qualitative behavior of the 2p and 2s level was similar for both systems. A local density functional (LDF) calculation of the electrostatic potential energy change as a function of alkali coverage allows us to separate out the contributions of initial and final state effects in the coverage dependence of these alkali core level binding energy shifts. Our results are more consistent with a covalent model of alkali-metal bonding.