Inverse photoemission from alkali-metal films

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Abstract
Inverse-photoemission spectra in the form of 10.5-eV isochromats have been recorded for thin films of Na, K, Rb, and Cs on a Cu(100) substrate. At submonolayer coverages the image-potential states of the substrate are first displaced and then quenched. Multilayer polycrystalline films yield inverse-photoemission spectra dominated by a steeply rising background attributed to plasmon creation. The different form of the inelastic backgrounds in photoemission and inverse photoemission from the alkali metals is rationalized in terms of the energy dependence of plasmon-scattering cross sections. The possibility of coherent processes is discussed. Expected structures due to the unoccupied d bands are absent or weak.