Linear and nonlinear response of alkali-metal adlayers on metal surfaces to a static electric field

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Abstract
The linear and nonlinear response of alkali-metal adlayers on metal surfaces to a static electric field is studied within the local-density-functional theory as a function of the adatom coverages. Hexagonal Na layers with varying lattice constants are used as adlayers, and the metal substrate is represented by the semi-infinite jellium. The predictions of these first-principles electronic-structure calculations are compared with those of quasi-one-dimensional models where the positive ions of the alkali-metal adlayer are represented by a thin jellium model. For coverages above the work-function minimum, the linear and nonlinear response properties, which depend on the lateral average of the induced electronic density, are remarkably similar in both calculations. In particular, the position of the image plane and the nonlinear moment which determines the perpendicular second-harmonic surface polarization, are well reproduced within the jellium-on-jellium model. At coverages less than the work-function minimum, this model ceases to be realistic and the response properties strongly reflect the atomic character of the adsorbate.