ADSORPTION OF INTERACTING ATOMS TO (100) SURFACES MODEL-CALCULATIONS - HARD-CORE AT NN AND FINITE INTERACTIONS UP TO 3RD NEIGHBOR DISTANCE

Some species of atoms adsorbing to (100) surfaces of metals show a very strong repulsion at nearest neighbor (nn) position and finite interactions over larger distances. In this contribution adsorption isotherms are calculated for the case of infinitely strong nn repulsion, finite second neighbor repulsion and third neighbor attraction using an extended Ising model on a periodic lattice,of 50x50 sites. They show the typical behavior corresponding to mixed interactions. Although the detailed discussion of critical phenomena is postponed, a tricritical point emerges where the phase transitions change from second to first order. Moreover within a certain parameter range the isotherms show an unusual asymmetry between the low-c(s) (c(s) < 0.25) and the high-c(s) (0.25 < c(s) < 0.5) part closely resembling experimental equilibrium adsorption isotherms for O/Ni(100). From the comparison between theory and experiment values for the oxygen interaction potentials on Ni(100) are deduced.