INTERACTION OF CL-2 WITH THE AU(111) SURFACE IN THE TEMPERATURE-RANGE OF 120-K TO 1000-K

The interaction of gaseous chlorine (Cl2) with a Au(111) single-crystal surface over the temperature range of 120-1000 K has been examined by using Auger electron spectroscopy (AES), X-ray and UV photoelectron spectroscopy (XPS and UPS), low-energy electron diffraction (LEED), temperature-programmed desorption (TPD), and work-function change (DELTAphi) measurements. Chlorine dissociatively adsorbs at all temperatures above 120 K. The Cl2 dissociative sticking coefficient decreases as the exposure temperature increases above 120 K, yielding an apparent activation energy of -0.2 kcal/mol for this process. No stable molecularly adsorbed Cl2 state was found at 120 K, which is just above the temperature required to desorb condensed Cl2 films. The work function increases with Cl2 exposure and reaches a maximum value of DELTAphi = 1.2 eV at 120 K and DELTAphi = 0.9 eV for the 500 K dose, indicating occupation of adatom sites on the Au(111) surface. An upper limit for the saturation coverage of Cl atoms, theta(Cl)sat, produced by Cl2 exposure under ultra-high vacuum conditions was estimated from AES calibration as theta(Cl)sat less-than-or-equal-to 2.9. A (square-root 3 x square-root 3)R30-degrees LEED pattern was observed for the first time at temperatures below 230 K. A structural model is proposed for the LEED pattern in which there are four Cl atoms per unit cell with atomic radii of 1.24 angstrom, giving theta(Cl) = 1.33. Loss of Cl from the surface starts to occur upon heating the Au substrate to 600 K, due primarily to desorption of Cl-containing species rather than diffusion into the bulk. Two desorption states, at 790 and 640 K, were observed in TPD which differ in their Cl+/Cl2+ ratio as detected by the mass spectrometer. We assign these peaks as due to desorption of Cl(g) and Cl2(g), respectively, with desorption activation energies estimated to be 48 and 39 kcal/mol, respectively. No desorption of AuCl(x) species was observed. The Cl(2p) core-level peak at 197.3 eV binding energy (BE) in XPS did not shift with Cl2 exposure or temperature. UPS identified chlorine-derived peaks at 1.9, 4.4, 5.5 and 6.9 eV BE, but no peaks were assigned to gold chlorides. Since only small changes in the chemical nature of Au surface atoms were observed in XPS and UPS, the interaction between Cl2 and Au(111) forms chemisorbed chlorine adatoms, Cl(a), and not a surface chloride compound under our conditions. However, the work function and AES uptake curves exhibited local minima before the onset of the desorption, which might be interpreted as some incorporation of chlorine into the surface layer at large theta(Cl). We have summarized the thermochemistry of chlorine adsorption on Au(111) and estimate that the dissociation energy of the Cl-Au bond, D(Au-Cl) is about 54 kcal/mol. This is at least 20 kcal/mol smaller than on Ag.