Successive multilayer desorption from ultrathin alkali-metal films studied with helium-atom scattering

Using high-resolution energy-resolved helium-atom scattering the temperature-dependent behavior of phonons in ultrathin potassium films has been investigated. The experiments show that “organ pipe” modes with frequencies determined by the local film thickness dominate the phonon dispersion curves in films of thicknesses below ten monolayers. The vanishing of certain film modes and the emergence of new modes at sufficiently high temperatures indicates a partial and, eventually, a complete thermal desorption of the film. An evaluation of the data demonstrates that the desorption proceeds in a layer-by-layer mode. At temperatures below the onset of desorption annealing effects have been measured, also using the spectroscopy of different film modes.