A NEW ANALYSIS OF IMAGE CHARGE THEORY

Ab initio cluster model wavefunctions for Al and Pt surfaces have been used to investigate the response of metal surfaces to the presence of point charges above the surface. The dipole moment curves, mu(z), as a function of the distance of the point charge above the surface have been used to characterize the metallic response. There are three regio ns where the behavior of mu(z) is different. In the region where z > s imilar to 15-20 bohr, the asymptotic behavior given by image charge th eory holds. In the region where z < similar to 4 bohr, the well known failure of the asymptotic formula is found; this reflects the fact tha t the point charge penetrates the metal charge density. In an intermed iate region, similar to 4 < z < similar to 15-20 bohr, a new and unexp ected behavior has been found. The magnitude of the slope of the dipol e moment curve is significantly larger than the value of the point cha rge! In this region the metallic response is strongly dependent on the distance of the point charge from the metal surface. Further, the mag nitude of the slope is quite different for positive and negative point charges indicating that the metallic response has important contribut ions from non-linear terms. (C) 1998 Elsevier Science B.V. All rights reserved.