LOW-ENERGY ELECTRON-DIFFRACTION ANALYSIS OF THE (ROOT-7X-ROOT-7) R19.1-DEGREES-S ADSORBATE STRUCTURE ON THE PD(111) SURFACE

We performed a detailed low-energy electron-diffraction (LEED) analysi s of (square-root 7 x square-root 7)R19.1-degrees-S on Pd(111) and a r e-analysis of (square-root 3 x square-root 3)R30-degrees-S on Pd(111). Sulphur adsorption on the Pd(111) surface involves a (square-root 3 x square-root 3)R30-degrees structure at room temperature, which by fur ther exposure to H2S and heating to temperatures of approximately 450 K transforms irreversibly into a structure with (square-root 7 x squar e-root 7)R19.1-degrees periodicity. There are distinct differences in the atomic structure of the high and the room temperature sulphur phas es. For the (square-root 3 x square-root 3)R30-degrees phase, we find the S atoms occupy fcc-hollow sites in agreement with a previous LEED analysis; we find, however, in contrast to the earlier analysis, adsor bate-induced relaxations of the Pd(111) surface. The present analysis permits one to reject all the simple sulphur overlayer geometries cons idered for the (square-root 7 x square-root 7)R19.1-degrees structure and suggests that this phase comprises a mixed palladium-sulphur overl ayer. However, a final conclusion regarding the exact atomic structure of this phase cannot yet be reached on the basis of the obtained leve l of agreement with the experimental LEED data.