CDI2 SINGLE-CRYSTAL GROWTH ON CU(111) - ADSORPTION, DESORPTION AND FORMATION OF A CHEMISORBED CDI1 PHASE

The adsorption and desorption of CdI2 on Cu(lll) has been studied usin g Auger electron spectroscopy and low-energy electron diffraction. Mul tilayer adsorption occurs at room temperature via a layer-by-layer gro wth mechanism. The first chemisorbed layer, consisting of 1/3 ML of io dine and 1/6 ML of Cd, has a (root 3 x root 3)R30 degrees structure. S ubsequent layers, consisting of hexagonal I ... Cd ... I sandwich laye rs, grow as single crystal with the hexagonal CdI2 unit mesh vectors ( 4.24 Angstrom in length in the bulk material) parallel to and coincide nt with the Cu(111) root 3 distance (4.42 Angstrom in length). The '(r oot 3 x root 3)R30 degrees diffraction beams therefore persist even fo r very thick layers of CdI2 as they are the first-order diffraction be ams of the CdI2 single crystal. The CdI2 multilayers desorb between 38 0 and 410 K, leaving a surface having the stoichiometry CdI1, consisti ng of 1/3 ML of iodine and 1/3 ML of Cd, which exhibits a good (root 3 x root 3)R30 degrees structure in which the Cd probably lies between the iodine adlayer and the copper surface. Between 420 and 450 K, cadm ium is lost from this surface, presumably by desorption, leaving a (ro ot 3 x root 3)R30 degrees structure composed of 1/3 ML of pure iodine. A structure is proposed for the CdI1 adlayer and the mean free paths of the Auger electrons through the CdI2 multilayer are calculated.