Sodium and potassium monolayers on Be(0001) investigated by photoemission and electronic structure calculations

Abstract

Photoemission spectra show that Be(0001) 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 $2\ifmmode\times\else\texttimes\fi{}2$ order for K while Na forms an incommensurate close-packed 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 Be(0001) and $p(2\ifmmode\times\else\texttimes\fi{}2)\text{K}/\text{Be}(0001)$ 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.