AVS 45th International Symposium
    Surface Science Division Monday Sessions
       Session SS1-MoM

Paper SS1-MoM2
Unoccupied States vs Coverage in Alkali Metals on Mo(110) by Auger Decay, NEXAFS, and Bandmapping

Monday, November 2, 1998, 8:40 am, Room 308

Session: Issues in Surface Electronic Structure
Presenter: E. Rotenberg, Lawrence Berkeley National Laboratory
Authors: E. Rotenberg, Lawrence Berkeley National Laboratory
C.M. Lee, Lawrence Berkeley National Laboratory
S.D. Kevan, University of Oregon
Correspondent: Click to Email
In recent years, the nature of the chemical bonding between alkali metals (AMs) and metal substrates has been intensely debated. For many alkalis on metals, the workfunction rapidly falls with coverage, reaching a minimum at about 1/2 monolayer before recovering somewhat up to saturation. The key issue is whether this work function change directly reflects a strong change in the charge distribution around the AM, as proposed by Langmuir and Gurney (LG) over 60 years ago, or whether the charge distribution somewhere else (in the substrate or in the space between adsorbate and substrate) dominates the workfunction change. Using localized, core-hole processes at the AM (near edge xray absorption fine-structure spectroscopy (NEXAFS) and Auger decay) we can now infer the alkali s-level occupation as a function of coverage, and correlate this with other measurements such as bandmapping and workfunction measurements. Measurements were performed for Li or K on Mo(110). At low coverages (below about ~0.1 ML) we find no occupied states at the AM atoms. At somewhat higher coverages, we observe a reoccupation of the AM valence orbital. This reoccupation, however, appears largely complete before the workfunction minimum occurs. Therefore, we conclude that there is a significant change in AM valence occupation with coverage, roughly following the LG model (donation of charge from AM to the substrate at low coverage, followed by backdonation at higher coverages). However, the workfunction change cannot be explained only by our measurements of the AM valence occupation, but instead has additional contributions from redistribution within the substrate.