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 C e-Pt compounds were formed. For films with initial coverages greater t han 3.5 ML, a hexagonal pattern with periodicity nearly twice that of Pt(111) appeared after annealing. For this 1.96X1.96 structure, the pe riodicity 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 fi lms having initial coverages from 0.9 to 1.8 ML, a 1.94X1.94 pattern w ith 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 wa s observed with valence around 3.09. Studies of the take-off angle dep endence of the emission indicate that the valence has the same value n ear the surface as in the bulk. We have also performed CO and O-2 adso rption studies and found extremely low sticking coefficients for these molecules on the 1.96X1.96 CePt2.23 surface. Since these well-charact erized single-crystal Ce compound films are highly suitable for photoe mission experiments and adsorption studies, we have attempted to formu late empirical rules for predicting which compound species will be gen erated by similar means.