SIMULATIONS OF THE ORDER-DISORDER PHASE-TRANSITION AND THERMAL-DESORPTION ON A SQUARE LATTICE BEYOND THE LATTICE-GAS APPROXIMATION

Employing Monte Carlo simulations, we calculate a phase diagram and th ermal desorption spectra for adsorption on a square lattice in the fra mework of the model laking into account the shifts of chemisorbed part icles from equilibrium positions. At realistic values of the activatio n barrier for surface diffusion, A = 3-20 kcal mol(-1), the results ob tained are surprisingly close to those predicted by the standard latti ce-gas model. In particular, the critical temperature, T-c, for the or der-disorder phase transition calculated for A = 20 kcal mol(-1) is th e same as that given by the standard model. With decreasing A down to 3 kcal mol(-1), the drop in T-c is only similar or equal to 10%. In th e thermal desorption spectra, the decrease of A results in decreasing the maximum intensity of the low-temperature peak, but this effect is rather weak. (C) 1997 Elsevier Science B.V.