A molecular beam study of nonlinearity in the CO-induced surface restructuring of Ir{100}

The kinetics of CO chemisorption on both the (1 x 5) and (1 x 1) surfaces of Ir{100}, including the CO-induced surface restructuring process, have been studied by measuring the sticking probability as a function of the surface temperature and beam flux. Due to competition between desorption from the (1 x 5) phase and growth of(1 x 1) islands, the sticking probability on the initial (1 x 5) surface is strongly flux-dependent at surface temperatures T-s in the range 480 less than or equal to T(s)less than or equal to 510 K. It is shown that this is due to a strongly nonlinear dependence of the (1 x 1) growth rate on the local CO coverage on the (1 x 5) substrate, with an apparent reaction order of around 5. Desorption energies and pre-exponentials of desorption for CO from both the (1 x 1) and (1 x 5) surfaces have been determined by means of a modified lifetime measurement technique. Equilibrium coverages as well as isothermal desorption rates of CO were determined for both surface phases. The zero coverage desorption energy of CO from the (1 x 1) substrate is 196 +/- 5 kJ/mol and from the (1 x 5) surface it is around 150 kJ/mol. This difference in adsorption energies is the driving force for the CO-induced (1 x 5) to (1 x 1) phase transition. TEAS data show that the local CO coverage on the growing (1 x 1) islands during the phase transformation is 0.5 ML. (C) 1998 American Institute of Physics. [S0021-9606(98)70348-4].