A MOLECULAR-BEAM STUDY OF THE H-2-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 recon structed (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 deso rption behaviour is observed In the case of the (1 x I) surface. Adsor ption on (1 x 5) does not lead to a complete lifting of the reconstruc tion, as confirmed by low-energy electron diffraction measurements and the H-2-D-2 exchange reaction. However, both techniques support the f inding that the extent of the (1 x 1) transformation is improved bg su ccessive annealing and cooling cycles performed in the presence of the impinging beam. The exchange reaction, studied over a sample temperat ure of 250 to 1000 K, proved to be very useful in the identification o f structural changes in the iridium surface under the influence of hyd rogen. The reactive sticking probability, s(r), shows a strong hystere sis 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 deuteriu m 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.