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 r egime (0.015<theta<0.10) to a lattice-gas model making the quasi-equil ibrium approximation and using a transfer matrix grand partition funct ion. The parameters determined from this analysis are compatible with a wide range of experimental data for this system, and indicate a syst em that is weakly interacting outside a short-range hard-wall repulsio n. Low-coverage Arrhenius parameters were determined by fitting the de sorption waveforms below theta=0.015. The Arrhenius prefactor and acti vation energy for desorption are determined to be log(A)=15.1+/-0.7 an d E(d) = 130+/-5 kJ mol(-1), respectively, in good agreement with prev ious 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 qua litatively explain the differences between these two systems.