The adsorption geometry of sulphur on Ir{1 0 0}: A quantitative LEED study

A quantitative low energy electron diffraction (LEED) analysis has been performed for the p(2 × 2)-S and c(2 × 2)-S surface structures formed by exposing the (1 × 1) phase of Ir{1 0 0} to H₂S at 750 K. S is found to adsorb on the fourfold hollow sites in both structures leading to Pendry R-factor values of 0.17 for the p(2 × 2)-S and 0.16 for the c(2 × 2)-S structures. The distances between S and the nearest and next-nearest Ir atoms were found to be similar in both structures: 2.36 ± 0.01 Å and 3.33 ± 0.01 Å, respectively. The buckling in the second substrate layer is consistent with other structural studies for S adsorption on fcc{1 0 0} transition metal surfaces: 0.09 Å for p(2 × 2)-S and 0.02 Å for c(2 × 2)-S structures. The (1 × 5) reconstruction, which is the most stable phase for clean Ir{1 0 0}, is completely lifted and a c(2 × 2)-S overlayer is formed after exposure to H₂S at 300 K followed by annealing to 520 K. CO temperature-programmed desorption (TPD) experiments indicate that the major factor in the poisoning of Ir by S is site blocking.