Ž . Ž . The effect of the formation of p 2 = 2 and c 2 = 2 adsorption structures on the O sticking probability was studied by 2 Ž . the Monte Carlo simulation of the O chemisorption on a 100 metal surface. The model used in the simulation took into 2 account the direct and indirect adsorption pathways and repulsive lateral interactions in the adsorption layer. The ratio Ž . between the activation energy of adsorption via the direct and indirect pathways E rE determines the character of the dir indir
Ž . structural transformations in the adsorption layer, which in turn determines the type of the S u dependence. At E rE -7-8, the direct pathway predominates over the indirect one. In this case, a lot of small adsorption islands dir indir Ž . Ž . nucleate to form a disordered p 2 = 2 and c 2 = 2 adsorption layers, i.e. Langmuir adsorption is observed, and S smoothly decreases with the u growth. At E rE ) 7-8, the indirect pathway predominates over the direct one. In this case, after dir indir Ž . Ž . slow nucleation, the adsorption islands grow quickly to form ordered p 2 = 2 and c 2 = 2 adsorption layers, i.e. the Ž . island-mediated adsorption is observed and S u dependence passes over a maximum.