ORIENTATIONAL ORDERING OF A CESIUM MONOLAYER ON GRAPHITE

Orientational ordering has been observed by low-energy electron diffraction for structures close to (2 x 2) in an overlayer of cesium absorbed on the graphite (0001) surface at liquid-nitrogen temperature. The relative orientation of the Cs overlayer to the graphite substrate varies from about 5-degrees to 0-degrees and the lattice misfit changes from about 27.6% to 0% as the coverage of Cs on the graphite surface is increased. For the same lattice-parameter misfit the rotation angles in the structure are almost half of those in the rare-gas physisorbed systems on graphite (0001), the Na chemisorbed on Ru(001), and the Cs planar layer intercalated in the high-stage graphite intercalation compounds. The (2 x 2) family of structures for the Cs-chemisorption-on-graphite system is consistent with the Novaco-McTague theory of orientational ordering, but there is a stronger bond strength between adatoms in the Cs/C overlayer than in the above other three systems.