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 ph otoemission spectroscopy. For increasing Na coverage, the core level b inding 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 c ompletion of the first monolayer. The qualitative behavior of the 2p a nd 2s level was similar for both systems. A local density functional ( LDF) calculation of the electrostatic potential energy change as a fun ction of alkali coverage allows us to separate out the contributions o f initial and final state effects in the coverage dependence of these alkali core level binding energy shifts. Our results are more consiste nt with a covalent model of alkali-metal bonding.