Compressibility of CO adsorbed on Ni(1 1 1) from 10−6 mbar to 1.2 bar ambient CO pressures investigated with X-ray diffraction

The adsorption of CO on Ni(1 1 1) surfaces at room temperature has been investigated under equilibrium conditions in ambient CO gas pressures ranging from 10−6 mbar to 1.2 bar with surface X-ray diffraction. Within this pressure range, ordered structures close to √7×√7R19.1° have been observed to occur. Their lattice parameter was found to change reversibly with the gas pressure by 7.6% within the nine decades pressure range investigated. Ab initio total energy numerical calculations reveal that the compressibility of the adlayer is consistent with a pairwise repulsive force of ∼0.1 eV/Å between neighboring CO molecules.