Frequency-dependent photoemission from aluminium

A calculation of the photoemission from Al (100) and (111) surfaces, which includes band-structure effects but neglects screening of the photon field, gives good agreement with experimental results at fixed photon energy; it does not reproduce the observed variation in emission intensity from a fixed initial state as a function of photon frequency. Treating Al as a free-electron gas with a frequency-dependent local dielectric function is also unsatisfactory. However, using the simple hydrodynamic method for non-local screening at a metal surface, in which the surface polarisation charge is treated more realistically, the authors obtain good agreement with the frequency dependence of photoemission both from the Al(100) surface state, and from the Fermi level. This shows enhancement in photoemission below the bulk plasmon frequency, and a minimum at omega p.