Molecular and dissociative adsorption of 1-bromo-2-chloroethane on Cu(111)

The adsorption of 1-bromo-2-chloroethane (BCE) on Cu(lll) in the temperatur e range 100-300 K has been studied using ultraviolet photoelectron spectros copy (UPS), work function measurements, low energy electron diffraction, an d line of sight temperature programmed desorption (LOSTPD). At 300 K adsorp tion occurs dissociatively forming ethene (which desorbs) and a (root 3 x r oot 3)R30 degrees-Br/Cl chemisorbed structure which increases the work func tion of the surface by 0.75 eV. At 100 K BCE adsorption is molecular, with a decrease in work function of 0.55 eV for the first monolayer. By combinin g UPS and LOSTPD to study the adsorbed and gas phase products generated by heating the monolayer and multilayer surfaces formed at 100 K, several stat es of BCE on the Cu(lll) surface have been observed. The A (first layer), B (2nd layer) and C (>2 layers) states correspond to BCE adsorbed prior to a ny dissociative reaction with the copper surface. Reaction of BCE with the Cu(lll) surface then forms three more states. The A' state isa submonolayer of BCE which has been stabilised by chemisorbed halogen products after par tial thermal decomposition (130 K) of a single monolayer of BCE. The A' sta te undergoes complete dissociation on heating to 150 K. Two molecular BCE d esorption states, the B'1 and B'2, are observed on heating multilayers of B CE. They are thought to be due to BCE in the second layer (originally the B state) which is stabilised by chemisorbed halogen products formed by decom position of the underlying first layer (originally the A state). The possib ility of the B' states having a gauche conformation is discussed. (C) 1999 Elsevier Science B.V. All rights reserved.