Phase diagram and overlayer structures for CO on Rh(110)

Abstract

Low energy electron diffraction (LEED) patterns have been recorded for CO on Rh(110) and analyzed as a function of sample temperature or CO dose using a video-LEED system. The results provide a diagram of boundaries and existence regions for overlayer phases. The overlayer is a disordered lattice gas at absolute CO coverages (θ CO) below about 0.08 for all temperatures. A pattern characterized by splitting of c(2 × 2) spots appears most intense for θ CO ≈ 0.3–0.6 below 180 K. The “split” c(2 × 2) pattern converts to a pattern with diffuse, broad c(2 × 2) spots above 180 K. Overlapping diffraction features from “(3 × 2)”, “(4 × 2)”, and “(5 × 2)” symmetries are observed for θ CO above about 0.62 and below 240 K. The “(5 × 2)” predominates by θ CO ≈ 0.8. With further increase in θ CO, (2 × 1) LEED spots appear with the “(5 × 2)” features. A pure (2 × 1) pattern appears at monolayer θ CO. Overlayer structural models with CO molecules in short-bridge sites (off-top models) or tilted from on-top adsorption sites (tilt models) are proposed to account for features in the LEED patterns. Off-top models giving rise to pseudo-hexagonal ordering are favored for the “split” c(2 × 2) overlayer. The “c(2 × 2)” overlayer contains uniformly sized domains with ideal c(2 × 2) ordering and disordered domain walls. Increasing 3-, 4-, and 5-fold symmetry in off-top adlayers arises from uniform, commensurate compression along [110]. In tilt overlayer models by comparison, the “( n × 2)” structures contain CO molecules in rows with local (2 × 1) symmetry separated by 3, 4, or 5 unit cell spacings. A zig-zag tilt model for reconstruction in the (2 × 1) CO phase is proposed.