Adsorption of chlorine on clean and on oxygen preexposed Al(111)

The adsorption of Cl2 on clean and on O2 preexposed Al(111) has been studied using Auger and electron energy-loss spectroscopies, low-energy electron diffraction, temperature programmed desorption, and work function measurement. Near room temperature chlorine forms a disordered layer in a two-step process consisting of adsorption at surface sites, dominating at low exposure, followed by population of subsurface sites at higher exposure. Surface adsorption is characterized by a sticking coefficient of ∼0.1, an increase in work function of ∼1 eV, and a desorption peak at ∼200 °C. The subsurface phase exhibits a smaller sticking coefficient and work function change and no distinct desorption peak. Preadsorbed O blocks formation of subsurface species but not those formed in the initial adsorption step. Cl2 does not displace preadsorbed O, but O2 displaces some subsurface Cl. Chlorination is found to occur inhomogeneously rather than uniformly in a layer-by-layer mode.