Formation of and CO adsorption on an inert La-Pt(111) surface alloy

Thin La overlayers (up to approximate to 3 ML) have been deposited onto a P t(111) surface. We find that an ordered interface alloy is formed when the system is annealed above 800 K. A 2 x 2 overlayer structure was observed by low-energy electron diffraction. Photoemission reveals a substantial incre ase in the density of states near the Fermi level as a result of the alloyi ng reaction. When the as-deposited La-on-Pt(111) system was exposed to CO, and subsequently annealed, a desorption peak was observed to coincide with the alloying transition. The signature of this desorption peak was that of zeroth-order desorption. Thermal-desorption studies showed that CO is more weakly adsorbed on the La-Pt surface alloy as compared to the pure Pt(111) surface. A desorption energy E-d = 0.9 +/- 0.1 eV was estimated for CO on t he surface alloy in the limit of zero CO coverage. The desorption measureme nts are discussed in view of recent Monte Carlo simulations as well as calc ulations using the transfer-matrix method. [S0163-1829(99)03223-3].