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(lll) single crystal surface at room temperature at coverage greater than or equal to 0.5 ML. A background pressure of CO was maintained during imaging. The structures observed include a (2 x 1) and two forms of (root 7 x root 5)R19 degrees 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 x 2)-3CO, was found to be stable under CO pressures of at least 700 Torr. (C) 2000 Elsevier Science B.V. All rights reserved.