DISSOCIATIVE CHEMISORPTION OF O2 ON CU(110)

The initial sticking coefficient S0 for the dissociative chemisorption of O2 on Cu(110) has been measured as a function of translational energy, surface temperature and scattering azimuth using a molecular beam. Sticking is facile on this face, having an initial sticking coefficient S0 = 0.2 at 0.05 eV and rising to S0 = 0.8 at translational energies above 0.35 eV. The dissociation probability is independent of scattering azimuth and follows a normal (cos2theta(i)) energy scaling. At low beam energies the sticking coefficient shows a strong surface temperature dependence, S0 rising to approximately 0.6 at 100 K. For beam energies above approximately 0.2 eV the sticking coefficient is independent of surface temperature. This is interpreted in terms of two competing mechanisms, a trapping-desorption channel which dominates at low translational energies and low surface temperature (T(s)) and a direct, activated dissociative chemisorption channel which becomes important as the energy is increased above 50 meV. The trapping state is thought to be a weakly bound, physisorbed molecular state, while the weak temperature dependence at high beam energies indicates direct, activated dissociation with negligible kinetic competition due to desorption via an (unstable) molecularly chemisorbed state. The coverage dependence S(theta) changes from a linear dependence at high beam energy and surface temperature to a slower decrease with theta at low T(s), characteristic of an extrinsic precursor state. The trapping channel can be modelled as a function of theta and T(s) using a simple kinetic model for the precursors, based on the kinetic parameters obtained for the intrinsic precursor state and assuming local ordering of the adsorbate, even at the lowest surface temperatures (100 K).