Adsorbate-adsorbate interactions from statistical analysis of STM images: N/Ru(0001)

Atomic nitrogen on Ru(0001) was prepared by dissociative chemisorption of N₂ and studied by scanning tunneling microscopy (STM) at 300 K. Nitrogen occupies the hcp threefold hollow site and is imaged as a depression with a diameter of about 5 {}. Interactions between the adsorbed nitrogen atoms were obtained by statistical analysis of STM images, by extraction of the two-dimensional pair distribution function from the arrangement of the N atoms. Since the nearest-neighbor separations could be identified with atomic precision, the pair distribution function g and hence the potential of mean force Veff were obtained as a function of the discrete neighbor sites j up to the tenth nearest neighbor. A comparison with Monte Carlo calculations for balls with a hard-sphere potential provides information about the pair potential Vₚₐᵢᵣ(j): The nearest-neighbor site is strongly repulsive, the second-neighbor site is weakly repulsive, and the third-neighbor site is weakly attractive. These findings rationalize the absence of island formation and of a well-ordered 2×{}2 phase for the N/Ru(0001) system: At temperatures {≥}300 K the attractive interaction on the third-neighbor site is too weak, while at lower temperatures the diffusion barrier of 0.9 eV represents a kinetic obstacle. The fact that the range of the interaction is identical to the diameter of the N-atom features in the STM topographs is taken as evidence that the interaction is caused by substrate-mediated electronic forces.