STRUCTURE AND VALENCE OF THE CE/PT(111) SYSTEM

We have studied the growth of thin cerium films on Pt(111) with x-ray photoemission spectroscopy (XPS) and low-energy electron diffraction. No ordered structures were observed for the Ce films immediately after room-temperature deposition. After heat treatment to 770 K, ordered Ce-Pt compounds were formed. For films with initial coverages greater than 3.5 ML, a hexagonal pattern with periodicity nearly twice that of Pt(111) appeared after annealing. For this 1.96X1.96 structure, the periodicity is close to that of the compound CePt2, the XPS data show a stoichiometry of CePt2.23, and the observed Ce valence is 3.07. For films having initial coverages from 0.9 to 1.8 ML, a 1.94X1.94 pattern with small satellites around the main spots appeared; the Ce valence is 3.11 and the stoichiometry is CePt3. Between 2.1 and 3.2 ML, a 1.96X1.96 pattern with a superimposed 1.96X1.96 net rotated by 30 degrees was observed with valence around 3.09. Studies of the take-off angle dependence of the emission indicate that the valence has the same value near the surface as in the bulk. We have also performed CO and O-2 adsorption studies and found extremely low sticking coefficients for these molecules on the 1.96X1.96 CePt2.23 surface. Since these well-characterized single-crystal Ce compound films are highly suitable for photoemission experiments and adsorption studies, we have attempted to formulate empirical rules for predicting which compound species will be generated by similar means.