SUCCESSIVE MULTILAYER DESORPTION FROM ULTRATHIN ALKALI-METAL FILMS STUDIED WITH HELIUM-ATOM SCATTERING

Using high-resolution energy-resolved helium-atom scattering the tempe rature-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 phono n dispersion curves in films of thicknesses below ten monolayers. The vanishing of certain film modes and the emergence of new modes at suff iciently high temperatures indicates a partial and, eventually, a comp lete thermal desorption of the film. An evaluation of the data demonst rates that the desorption proceeds in a layer-by-layer mode. At temper atures below the onset of desorption annealing effects have been measu red, also using the spectroscopy of different film modes. [S0163-1829( 98)04807-3].