Dense structures formed by CO on Rh(111) studied by scanning tunneling microscopy

Scanning tunneling microscopy (STM) has been used to study the chemisorption of CO on a Rh(111) single crystal surface at room temperature at coverage ≥0.5 ML. A background pressure of CO was maintained during imaging. The structures observed include a (2×1) and two forms of (7×7)R19° that had not been previously observed. In contrast to the low coverage regime, above 3/7 ML, CO occupies both top and three-fold hollow adsorption sites. The highest coverage structure, (2×2)-3CO, was found to be stable under CO pressures of at least 700 Torr.