Interactions between adsorbed molecules: CO on Ni(111)

We report measurements of the isothermal desorption rate for the CO-Ni(111) chemisorption system. Values for microscopic lateral interaction parameters were determined by fitting the isotherms in the coverage regime (0.015<theta<0.10) to a lattice-gas model making the quasi-equilibrium approximation and using a transfer matrix grand partition function. The parameters determined from this analysis are compatible with a wide range of experimental data for this system, and indicate a system that is weakly interacting outside a short-range hard-wall repulsion. Low-coverage Arrhenius parameters were determined by fitting the desorption waveforms below theta=0.015. The Arrhenius prefactor and activation energy for desorption are determined to be log(A)=15.1+/-0.7 and E(d) = 130+/-5 kJ mol(-1), respectively, in good agreement with previous results. Comparisons will be made to the system CO-Pt(111) where appropriate. We discuss a simple model for substrate-mediated lateral interactions based upon relief of surface tensile stress which can qualitatively explain the differences between these two systems.