A molecular beam study of the H2-induced lifting of the Ir{100}-(1x5) reconstruction

Adsorption and desorption kinetics for hydrogen have been studied over Ir{100} by using molecular beam techniques. An almost linear coverage dependence is observed for the sticking probability on both the reconstructed (1x5) and the metastable (I x 1) surfaces. Thermal desorption experiments exhibit two distinct peaks, both with zero-order kinetics, for the initial(I x 5) surface. A single peak with second-order desorption behaviour is observed In the case of the (1 x I) surface. Adsorption on (1 x 5) does not lead to a complete lifting of the reconstruction, as confirmed by low-energy electron diffraction measurements and the H-2-D-2 exchange reaction. However, both techniques support the finding that the extent of the (1 x 1) transformation is improved bg successive annealing and cooling cycles performed in the presence of the impinging beam. The exchange reaction, studied over a sample temperature of 250 to 1000 K, proved to be very useful in the identification of structural changes in the iridium surface under the influence of hydrogen. The reactive sticking probability, s(r), shows a strong hysteresis over this temperature range due to adsorbate-induced restructuring of the surface from (1 x 5) to (1 x I). By using a modified lifetime measurement technique, the zero-coverage desorption energy of deuterium from the (1 x 1) surface has been determined as 98 +/- 14 kJ mol(-1); on the (1 x 5) surface at low coverage the heat was estimated as 85 kJ mol(-1). (C) 1998 Elsevier Science B.V. All rights reserved.