Electron delocalization by polar molecules:: Interaction of Na atoms with solid ammonia films studied with MIES and density functional theory

The interaction of Na and NH3 on tungsten was studied with metastable impact electron spectroscopy under UHV conditions. NH3(Na) films were grown at 90(+/-10) K on tungsten substrates and exposed to Na(NH3). No Na-induced reaction involving NH3 takes place. At small Na exposures a Na-induced shift of the NH3 spectral features is seen, in parallel with a decrease of the surface work function. At larger exposures three 3sNa-related spectral structures are seen, two of them at energetic positions different from that found for Na on metals or semiconductors. The main additional peak is attributed to delocalized Na species. A small additional feature is attributed to simultaneous ionization and excitation of partially ammoniated Na-2 species. The results are compared with density functional theory calculations which suggest that the 3sNa emission at small exposures appears to originate mainly from delocalized 3sNa electrons; they are located far from the Na species and become stabilized by solvent molecules. When depositing NH3 molecules onto Na films, metalliclike Na patches and delocalized Na species coexist. The delocalization of 3sNa is seen up to T=130 K where the NH3 species desorb. (C) 2004 American Institute of Physics.