THE SURFACE-STRUCTURE OF 1-BROMO-2-CHLOROETHANE ON CU(111)

The structure of 1-bromo-2-chloroethane (BCE) molecularly adsorbed on Cu(111) and the adsorption sites of chemisorbed chlorine and bromine c oadsorbed with the BCE have been determined using the normal incidence X-ray standing wave (NIXSW) method, Two Bragg reflections, the (111) and ((1) over bar 11), were used to triangulate the positions of the c hlorine and bromine atoms in the BCE molecule and the chemisorbed spec ies. The temperature used during the study, 130 K, allowed partial dec omposition of the BCE to chemisorbed halogens and gaseous ethene. The resulting surface, consisting of similar to 0.05 ML each of chemisorbe d chlorine and bromine and similar to 0.20 ML of physisorbed BCE, was stable to further decomposition. The chemisorbed halogens adsorbed in the hcp hollow sites, while the BCE molecule bonded to the surface at an atop site via the bromine atom at a distance of 2.63 +/- 0.2 K from the surface plane of the copper. The molecule was tilted with the Br ... Cl distance at an angle of similar to 11 degrees to the surface, t he chlorine being located approximately over an hcp site at a distance of 3.48 +/- 0.2 Angstrom above the surface plane of the copper. The c oherent fractions for the bromine and chlorine in the BCE molecule ind icated that the molecule could undergo large amplitude vibrations both parallel to and perpendicular to the surface by pivoting about the br omine end. (C) 1997 Elsevier Science B.V.