Reduced oscillator strength in the lithium atom, clusters, and the bulk

Abstract

Absolute photoabsorption cross sections have been measured for small lithium cluster ions in the optical range and a significantly smaller oscillator strength than for sodium has been found. This reduction is reproduced in jellium type calculations only if nonlocal effects in the electron-ion interaction are included. It is shown that this reduction is an atomic property which persists throughout the cluster region and into the bulk regime, where it manifests itself as an increased effective electronic mass. The optical spectra of the closed shell clusters are in good agreement with calculations based on the nonlocal jellium model. The smallest cluster, ${\mathrm{Li}}_{4}^{+},$ is well described by a quantum chemical calculation.