Charge transfer and positron states at alkali-metal-covered nickel surfaces

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Abstract
We report calculations of the positron surface state on clean and alkali-metal- (Na,Cs) covered Ni surfaces. It is shown that the alkali adsorption increases the surface-state binding energy. With the lowering of the electron work function, this leads to positronium desorption energies which are weakly dependent on the alkali coverage, in agreement with the recent experiments of Gidley, Köymen, and Capehart. It is shown that the positron becomes mainly localized between the substrate and the overlayer, and that the lifetime is shorter for positrons on the alkali-metal-covered surface.