Regular Mixing in a Two-Dimensional Lattice System: The Coadsorption of N and O on Ru(0001)

The coadsorption of N and O on Ru(0001) has been studied by scanning tunneling microscopy; by this technique we can distinquish between the two atomic adsorbates. N and O form a dense intermixed $2\times 2$ phase in equilibrium with a dilute lattice gas. The two-dimensional “vapor pressure” of N, i.e., its concentration in the lattice gas, has been determined for various ratios of N and O in the dense phase by adjusting the total coverage of N and O. The resulting vapor pressure curve indicates a positive enthalpy of mixing, due to relatively weak N-O interactions in the dense phase.