On the Ionization Potential of Small Metal and Dielectric Particles.

The ionization potential of small metal and dielectric spheres is considered in different frameworks: classical, semiclassical and quantum mechanical density functional approach. Classical calculations give conflicting results, and the generally accepted result for the ionization potential of a metal sphere of radius R: W sub I (R) = bulk work function + (3/8)q sup 2 /R is shown to be wrong, resulting from using the classical image potential too close to the metal surface. Using appropriate cut-off to the image potential, the result W sub I (R) = bulk WF+(1/2)q sup 2 /R (previously obtained from solvation energy considerations) is recovered. Experimental results on relatively large particles are in agreement with the latter result. For very small clusters, deviations of experimental results from this classical behavior are shown by a density functional calculation to arise from quantum mechanical effects. These are first the spilloff of the electronic wavefunctions beyond the cluster edge and secondly from exchange and correlation contributions.