Electron spectroscopy of alkali metal surfaces by deexcitation of metastable He atoms

• Published in: Surface science 1987-02, Vol.180 (1), p.187-202
• Main Authors: Woratschek, B., Sesselman, W., Küppers, J., Ertl, G., Haberland, H.
• Format: Article
• Language: English
• ISSN: 0039-6028; 1879-2758
• DOI: 10.1016/0039-6028(87)90043-4

Auger deexcitation of metastable He atoms yields electron energy distributions arising from the outermost atomic layers which may be compared with corresponding UPS data. This technique exhibits a much higher sensitivity for the s-derived valence states of the alkali metals than UPS. The recorded spectra do, however, not directly reflect the surface density of valence states, but are strongly modified by matrix element effects. A simple model based on tunneling of the valence electrons of a free-electron gas metal through a rectangular potential barrier into the He core yields energy distributions which can be fitted very well with the experimental data, and in particular it reproduces the observed exponential decrease of the emission intensity towards higher binding energies. The mean distance 〈 d AD〉 from which deexcitation predominantly occurs is the only adjustable parameter of this model, and the resulting values for 〈 d AD〉 are in the range of 3–5 Å from Li to Cs as expected. Singlet He ∗ atoms are converted to about 95% into the triplet state from where deexcitation occurs. The rate constant for this spin-flip process is estimated to be of the order of 10 14 s −1. Characteristic differences exist between the spectra of bulk alkali metals and of adsorbed monolayers.