Static Dipole Polarizability of Free Alkali Clusters

Abstract

To what extent do clusters made of only a few alkali-metal atoms behave as small metal particles ? The static electric dipole polarizability, α 0, is a basic observable for discussing this question since it is very sensitive to the effectiveness of the derealization of valence electrons, as well as to the structure and shape [1, 2]. Despite numerous investigations of alkali metal clusters, until very recently, polarizability measurements were only available for sodium clusters and for selected sizes of potassium clusters [3]. Nothing was known about lithium clusters. However, the static and dynamic response of lithium clusters to electric fields is in many respects the most interesting and puzzling. The polarizability of lithium atom is abnormally large as compared to the value of the bulk and to other alkali atoms. Moreover, several experiments have shown that the optical response of lithium clusters is significantly redshifted as compared to the classical prediction for a finite metallic sphere [4, 5]. This has been back up to non local effects in electron- ion interactions [6, 7, 8]. For a metallic sphere, the Mie frequency is directly related to the polarizability of the sphere, $$ \omega _M^2 = {{{e^2}N} \over {m{\alpha _{{\rm{c}}1}}}}. $$ (1) A direct experimental determination of the electric polarizability is therefore crucial both for understanding the size evolution and for the interpretation of the optical response.