THE ADSORPTION OF DEUTERIUM ON SULFUR-COVERED NICKEL(111) SURFACES

Deuterium adsorption on the (2 X 2), (root 3 X root 3)R30 degrees and (5 root 3 X 2) rectangular sulphur adlayers on a nickel(111) surface h as been studied by means of LEED and thermal desorption. Only the (2 X 2)S/Ni(lll) surface is able to adsorb significant amounts of deuteriu m and this results in a conversion of the (2 X 2) structure to a (root 3 X root 3)R30 degrees structure. The initial sticking probability du ring this process is about 10(-3) and the saturation coverage of deute rium is 0.13 ML. Deuterium desorption from this (root 3 X root 3)R30 d egrees co-adsorbed structure exhibits unusual kinetics, in which the p eak temperature does not vary over a significant range of initial cove rages, We attribute this to a second-order desorption step occurring f rom deuterium-containing islands. The simplest model that will fit the data involves islands which remain fixed in area during the desorptio n process and have a constant initial local coverage. The model yields an activation energy for desorption of D-2 of 58(+/- 10) kJ mol(-1). Some simple mechanistic models for the desorption kinetics are discuss ed.