Determination of lateral interactions between NO molecules on Rh(111)

. In this study, the STM was used to locally study the adsorption of NO on a Rh(111) single crystal. Three new structures were identified. At 200 K, patches of the (4 × 2)-2NO and an unreported (2 × 2)-2NO structure were found at about 0.50ML coverage. Higher exposure gave rise to a (2 × 2)-3NO structure with all molecules adsorbed in the hcp sites. At 5K, a (4 × 4)-1NO structure was observed. The large separation between the molecules can only arise from repulsive interactions over a distance of at least four times the lattice constant (11 Å). Here the interaction is estimated to be of the order of ~10K or ~0.1 kJ/mol. Dynamic Monte Carlo simulations taking into account adsorption, diffusion and pairwise interactions between adsorbates were used to fit the lateral interaction for the next-next nearest neighbor to the patched STM topograph at 200K for 0.50ML coverage. The value was determined to be 2 kJ/mol. Using pairwise interactions only, the (2 × 2)-3NO structure could not be explained. It might therefore be necessary to include three-particle interactions. Recent DFT calculations support this idea by showing attractive three-particle interactions.