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 o f Ir{100}, including the CO-induced surface restructuring process, have bee n studied by measuring the sticking probability as a function of the surfac e 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 th e initial (1 x 5) surface is strongly flux-dependent at surface temperature s 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-exponen tials 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. Equi librium coverages as well as isothermal desorption rates of CO were determi ned 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 sh ow that the local CO coverage on the growing (1 x 1) islands during the pha se transformation is 0.5 ML. (C) 1998 American Institute of Physics. [S0021 -9606(98)70348-4].