Electronic structure of the La–Pt(111) surface alloy

The electronic and structural properties of the La–Pt(111) surface alloy have been studied by angle-resolved photoemission, core-level spectroscopy and photoelectron diffraction. For low lanthanum coverages [<0.5 monolayers (ML)] annealing of as-deposited films at 900 K produces no structural change in the surface, as witnessed by low-energy electron diffraction (LEED) and core-level spectroscopy. The k∥ dispersion for such surfaces is very similar to the Pt(111) surface band structure. Annealing of surfaces with higher lanthanum coverages (>1 ML) produces a surface alloy which displays a (2×2) LEED pattern. The k∥ band structure for this alloy is changed dramatically compared with that of Pt(111). Photoelectron diffraction shows that lanthanum is situated in sites with six-fold rotational symmetry in the surface alloy. For the platinum sites our data are inconclusive, but in accordance with a structural model proposed for the Ce–Pt(111) surface alloy based on the layered Pt5Ce compound. We also present evidence for a surface-related state on Pt(111) that has not previously been discussed.