Photoemission from Alkali Metal Overlayers: Quantum Wells, Photoelectron Interference and Atomic Structure Changes
Angle resolved photoelectron spectroscopy is used to investigate alkali metal overlayers on noble metal surfaces. The work deals with interference in photoemission, bulk electronic structure and adsorption induced changes of atomic structure, respectively.
The relative importance of surface and bulk in photoemission has remained largely unexplored. For thick Li films, the Li bulk contribution is found to be suppressed in a narrow range of photon energies. This is ascribed to interference between bulk and surface emission in combination with a strong, plasmon induced, photon energy dependence of these contributions. For quantum well states (QWS) in thin (< 10 ML) Na layers on Cu(111) the photoemission cross section shows a strong oscillatory photon energy dependence that in qualitative terms may be explained by interference between the photoelectron waves from the back and front sides of the film. For 2 ML Na on Ag(111) a resonant QWS is observed via a drastic line shape change for the substrate bulk emission.
Via low energy electron diffraction and photoemission from QWS and the Li1s core level, changes of the atomic structure for Cu(111)/Li are monitored. Core level data confirms a previous suggestion that a substitutional (2(2) structure is formed when monolayer coverage is exceeded. A reversal of this adsorption-substitution structure change is observed upon a small exposure (around 0.1 L) of oxygen or water vapour. The observations suggest that the structure preferred is reversibly dependent on the surface content of neutral Li.
Photoemission spectra from thick (150 ML) and thin (1-17 ML) films of close packed Li on Cu(111) are recorded to determine the Li valence electron dispersion perpendicular to the (111)-direction. Due to an incomplete phase transition of bcc Li upon cooling below 75 K, its electronic structure is less well known than for other common metals. The present results show that films with close packed structure can be prepared thick enough for photoemission to be representative of bulk Li. It is found that the filled band width is 2.8 - 3.2 eV, which is appreciably smaller than the value obtained in energy band calculations (3.5 eV). The results furthermore indicate that the Fermi surface is closed.
quantum well states
angle resolved photoemission
surface electronic phenomena