LOW-ENERGY ELECTRON-ENERGY-LOSS SPECTROSCOPY OF SOLID FILMS OF ARGON - SURFACE AND BULK VALENCE EXCITONS

We report high-resolution electron-energy-loss (HREEL) spectra at low energy (0-20 eV) on solid multilayer films of Ar. The present results are compared with optical data of similarly deposited films as well as with electron-energy-loss spectroscopy of Ar in the gas phase. Relati vely wide and intense bands located at the energy of the n = 1, 1' and 2, 2' bulk excitons, which have been measured optically, are ascribed to the longitudinal branch of the bulk excitons. In the energy range of the n = 1, 1' surface excitons, up to four narrow peaks are resolve d with HREEL spectroscopy. At higher energy loss between the first and second bulk-exciton manifold, we have features ascribed to the n = 2 surface excitons along with several extra peaks, which have not been r eported previously. The observed trend is discussed in terms of Wannie r-type exciton series split by the spin-orbit interaction and originat ing from the surface of the solid.