CLOSE PACKING IN 2D LAYERS - THE STRUCTURE OF A CIS-1,2 DICHLOROETHENE MONOLAYER ON GRAPHITE

Preliminary results are presented for a single crystalline phase of ci s-1,2 dichloromethane monolayer adsorbed on graphite. No evidence was found of other phases before bulk condensation sets in. A possible str ucture for this adsorbed layer is proposed, determined by X-ray diffra ction measurements. The experimentally determined unit cell is incomme nsurate with respect to the underlying substrate and rectangular, with lattice parameters a = 11.91 angstrom, b = 5.36 angstrom and nu = 90- degrees. There are two molecules per unit cell arranged to give chains with space group cm, and all the molecules lie approximately flat on the surface. The 2D packing coefficient of the layer is determined to be 0.91, the same as the value expected for close packed spheres (0.91 ). The structure of this monolayer is discussed with respect to the cl ose packing arguments of Kitaigorodskii.