Ethene stabilised by halogens on Cu(111)

Temperature-programmed desorption studies have been carried out for a range of initial coverages of ethene adsorbed on equimolar chlorine and bromine covered Cu(111) surfaces (0-0.33 ML total halogen coverage) at 100 K. For the clean surface, ethene had an activation energy for desorption of 29 +/- 2 kJ mol(-1) at zero coverage, which decreased for increasing coverage, dropping to similar to 27 +/- 2 kJ mol(-1) for the highest coverage attainable at 103 K (using a fixed pre-exponential factor of 1 x 10(13) s(-1)). Chemisorbed chlorine and bromine stabilised the adsorbed ethene, the maximum effect occurring at 0.17 ML of Cl/Br where the sticking probability increased to 1.4, and the saturation coverage to 2.5 times that of the clean surface at 103 K. At a combined Br/Cl coverage of 0.25 ML, a particularly stable surface was formed for which the activation energy for ethene desorption was almost constant at 35 +/- 2 kJ mol(-1). The ethene is thought to be coplanar with the halogens and bonded directly to the copper via its pi orbital.