The adsorption of benzene mono- and multilayers on Ni(111) studied by TPD and LEED

Benzene adsorption on Ni(111) has been studied by LEED and TPD for adsorption temperatures between 110 and 310 K. At initial coverages θ_C6D6 < 0.05 ML (relative to Ni) complete decomposition of benzene occurs upon heating above 650 K. At higher coverages (θ_C6D6 > 0.05 ML) molecular desorption precedes decomposition. The saturation coverage of chemisorbed benzene can only be achieved at adsorption temperatures below 270 K and reveals a sharp (√7×√7)R19.1° LEED pattern corresponding to a coverage of 0.143 ML. This LEED pattern disappears upon heating to 300 K; simultaneously a very sharp molecular benzene desorption peak is observed at 294 K, indicating that desorption is coupled to this order-disorder phase transition. A previously reported (2√3× 2√3)R30 ° structure for room temperature adsorption can only be observed by coadsorbing CO; the relative coverages in the coadsorbed benzene + CO layer are θ_C6D6 : theta;_CO = 1:2. Adsorption of benzene at 110 K leads to the formation of multilayers. Three distinct states, α₁, α ₂and α₃ can be resolved with desorption temperatures of 154, 136 and 146 K, respectively. The α₁ peak corresponds to the first physisorbed layer on top of the chemisorbed benzene, probably also oriented parallel to the surface. The α₃ peak exhibits typical zero-order desorption behaviour and is attributed to bulk sublimation. The α₂ peak seems to be due to a transition layer between the α₁ layer and bulk benzene. This behaviour is similar to that observed before on Ru(001). For chemisorbed and physisorbed benzene we compile and compare the results on other surfaces.