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Ultrathin Na films and islands on graphite realized prototype simple metal quantum wells with all valence electrons confined within boundaries well defined on the atomic scale. This is shown by angle-resolved photoemission and scanning tunneling microscopy data, which give unique information about electron dispersion, hole lifetimes, and Fermi wavelengths.

Photoemission from quantum well states show that K films can be grown nearly layer by atomic layer on a cooled graphite substrate. The data also indicate that this growth is preceded by the formation of one intercalated monolayer of K metal. The successive steps of intercalation and adsorption means that the properties are more intricate than suggested by previous results, which have established K/graphite as a prototype metal/ semimetal adsorption system.

Photoemission spectra show that Be͑0001͒ surface states shift to lower energy with increasing Na or K coverage in the monolayer range. At an intermediate monolayer coverage a quantum well state appears near the Fermi edge and shifts to lower energy reaching saturation energy at full monolayer coverage. At full monolayer coverage low energy electron diffraction shows 2 ϫ 2 order for K while Na forms an incommensurate closepacked structure aligned with the substrate. As a result of the different structures, the photoemission spectra show qualitative differences that are explained by diffraction. First-principles calculations for Be͑0001͒ and p͑2 ϫ 2͒K / Be͑0001͒ reproduce reasonably the measured energy shifts and dispersions. Spectra recorded for the shallow alkali-metal core levels show that the adlayer is inhomogeneous in an intermediate coverage range. While this is not noted when the valence state energies are measured a linewidth change observed for one of the surface states is ascribed to this inhomogeneity. It is suggested that the onset of inhomogeneity is associated with the occupation of states in the quantum well band. Occupation of these states, which are highly localized to the adlayer, gives metal character to the layer over an increasing area as the coverage is increased making the film homogeneous at high monolayer coverage. An anomalous emission line is observed for both Na and K as a low energy companion to the quantum well state line becoming increasingly separated from this as the coverage increases. We suggest that the satellite is due to an energy loss associated with collective oscillations in the overlayer.

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