Determination of adsorbate structures on metal surfaces: alkalis, rare gases, and hydrogen on tungsten

The information needed for characterization of adsorbate structures is discussed, and the experimental situation is summarized for three much studied prototype systems: the alkalis, rare gases, and H2 on individual crystal planes of W. While fairly complete pictures of these systems emerge, notable gaps and contradictions remain. The number of distinct states on the surface has not been established for the alkalis and the rare gases on most planes. LEED patterns for Cs and H2 on (100)W have not yet been correlated with amounts and binding states determined by other methods. The alkalis and rare gases are remarkably similar in dipole moment and relative binding energies on different crystal planes. Structures of these systems appear to be determined by substrate geometry, and bonding can be explained reasonably well on the basis of van der Waals and electrostatic forces. For hydrogen the number of states cannot be predicted from substrate geometry alone nor can the relative binding energies be explained from known forces. Still lacking for all of these systems are unequivocal determination of the adsorption sites and the charges on the adsorbates.