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

Thin La overlayers (up to ≈3 ML) have been deposited onto a Pt(111) surface. We find that an ordered interface alloy is formed when the system is annealed above 800 K. A 2×2 overlayer structure was observed by low-energy electron diffraction. Photoemission reveals a substantial increase in the density of states near the Fermi level as a result of the alloying 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 the surface alloy in the limit of zero CO coverage. The desorption measurements are discussed in view of recent Monte Carlo simulations as well as calculations using the transfer-matrix method.