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

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Abstract
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.