Overlayer structure and kinetic behavior of benzene on palladium(111)

The structural and kinetic properties of benzene on Pd(111) are studied at 200 K using temperature-programmed desorption. Benzene completely thermally decomposes on Pd(111) for coverages below 0.05 (where coverages are referred to the number of exposed palladium atoms on the (111) surface) but desorbs molecularly in two distinct states at higher coverages. This behavior is explained by invoking a repulsive molecular interaction between adjacent adsorbed benzenes and confirmed using Monte Carlo simulations of the thermal desorption spectra. Benzene adsorbs onto palladium with an initial sticking probability of unity, the overlayer saturating at a benzene coverage of 0.13. Further benzene adsorbs but with a much lower sticking coefficient (0.03) up to an ultimate saturation coverage of ∼0.22, the additional benzene desorbing at low temperature (∼280 K). This, along with results of experiments where the benzene overlayer is compressed using NO, indicates that this state is due to the presence of a tilted benzene layer.