Structure and surface reactions of iodine and cadmium iodide on fcc(111) metal surfaces
Structural studies using the synchrotron based Normal Incidence X-Ray Standing Wave (NIXSW) technique of the copper(III)- √3x√3))R30 grad.-Iodine and copper(111)- √3x√3)R30grad.-1/2(CdI2) surfaces are presented. For the copperiodine system, the iodine was shown to adsorb in a mixture of fcc and hcp hollow sites at a distance of 2.16 ± 0.05A from the copper surface, in a (√3x√3))R30 grad. mesh. The hollow site ratio observed was 50 ± 3 % in fcc sites and 50 ± 3 % in hcp sites. For the copper-cadmium iodide system, the iodine was again shown to adsorb in a mixture of the three fold hollows, at a slightly smaller distance of 2.10 ± 0.05A from the copper surface, again in a (√3x√3)R30 grad. mesh. The ratio of occupation of the hollow sites was determined to be 37 ± 3 % in fcc sites and 63 ± 3 % in hcp sites. The copper(111)-( √3x√3)R30 grad.-Iodine surface produced by annealing the copper(111)- 1/2(CdI2) surface, was shown to have a different ratio again, at 80± 3 % in fcc sites and 20 ± 3 % in hcp sites. Possible explanations for the changing ratios are discussed including sample temperature during surface preparation, step density of the crystal, co-adsorption of adsorbate or contamination and surface coverage. The cadmium in the copper-1/2(CdI2) surface was shown to be adsorbed randomly in a mixture of the three fold hollow sites, at 2.25 ± 0.05A from the copper surface. The ratio was found to be 48 ± 3 % in fcc sites and 52 ± 3 % in hcp sites. Both studies were found to be affected by the presence of non-dipole effects in the angular distribution of the core level photoelectrons used to collect some of the data. This caused incorrect values for the standing wave structural parameters to be determined, A novel experiment was performed using two analyser geometries which enabled the importance of including the non-dipole terms in the standing wave equations to be confinned. An updated version of the standing wave equations is presented which allows quantification of and correction for the non-dipole terms. The surface reactions of iodine and cadmium iodide on an aluminium(111) surface at room temperature are shown to result in etching of the surface and the production of aluminium iodide (A1I3). For both systems, iodine forms a close-packed chemisorbed layer that has a (..J7x-..J7)R19.1° symmetry, with an iodine coverage of 3/7 of a monolayer. For the cadmium iodide surface, the cadmium is proposed as being located randomly above the chemisorbed iodine layer. With the sample liquid nitrogen cooled to low temperatures, iodine produced physisorbed multilayers, and cadmium iodide adsorbs intact, but with no ordered growth. A novel technique, Line Of Sight Sticking Probability (LOSSP), which allows the measurement of sticking and reaction probabilities is presented and applied to the I/Al system. The initial sticking probability for iodine at 300 K was determined as 0.8 ± 0.1. Under steady state etching conditions at 300 K the overall reaction probability for I2 to form AlI3, was, Rss = 0.36 ± 0.07. The surface consisted of a majority of chemisorbed iodine, with a minority of coadsorbed AlI3, with a total iodine coverage of ~ 0.6 ML. The sticking probability of I2, to solid iodine at 103 K was measured as Sphys = 0.98 ± 0.02, while the sticking probability on the halogenated surface at 300 K was measured as ~ys > 0.8 ± 0.1 Variable temperature measurements gave an activation energy for the desorption of All, of approximately 57 kJmol-1.