Sodium-induced modifications in the electronic structure of the W(100) surface

Adsorption of sodium on the W(100) surface is studied at room temperature by angular resolved ultraviolet photoemission spectroscopy (ARUPS) and LEED. A reconstruction of the W(100) surface induced by sodium adsorption is observed at least at low coverage. The high-lying tungsten surface states situated at 0.4 eV below the Fermi level at normal emission ( Gamma ) suffer a continuous shift towards higher binding energies when sodium coverage increases. With a monolayer of sodium, the shift attains 2 instead of 1 eV for caesium. The low-lying surface state located at 4.2 eV below the Fermi level at Gamma shifts towards higher binding energies in contrast with caesium adsorption. These effects reveal a stronger sodium atom-substrate interaction than that which occurs in caesium. The description of the Cs/W(100) system may be extended to Na/W(100): the d(z2)-like surface states of W(100) hybridise with the sodium 3s valence states to form a polarised covalent bond.