Abstract

Metropolis Monte Carlo simulations were used to examine the stable structures, phase coexistence and mechanism of nucleation and growth of films of CO$\sb2$ molecules adsorbed on the (001) surface of NaCl. This was done for submonolayer, monolayer, and multilayer coverages. The results show that the monolayer has two stable structures, a (2 x 1) herringbone structure and a (1 x 1) structure while the bilayer system has three stable structures: the (2 x 1), the (1 x 1) and the c(2 x 2). The trilayer system adopts a c(2 x 2) structure owing to the instability of the (2 x 1) structure. Multilayers most likely grow in the Stranski-Krastanov fashion whereby multilayer islands on top of a monolayer adopt a bulk-like crystalline structure $\{c$(2 x 2)$\}.$ At low coverages the CO$\sb2$ molecules behave like a 2D gas which aggregates into low density islands but finds it hard to nucleate the denser 2D solid phase directly, even in the presence of a surface step. This suggests that the adsorption of CO$\sb2$ molecules on the low density islands nucleates the 2D solid phase. The 2D gas-solid phase coexistence was found to obey the (2D) van der Waals equation of state. The critical coverage $\Theta\sb{c}$ and temperature T$\sb{\rm c}$ were found to be 0.162 and 142.9K respectively, close to the theoretical values of $\Theta\sb{c}$ = 1/6 and $T\sb{c}=151.2$K. For the CH$\sb3$Br/LiF(001) system, submonolayer and monolayer coverages were examined. The results show that for low coverages the molecules are tilted by 60$\sp\circ$ from surface normal while at monolayer coverage a stable (2 x 1) herringbone structure was found with a molecular tilt of 39.2$\sp\circ$ in agreement with experiment. An alternative incommensurate monolayer structure was found where the molecules are antiferroelectrically ordered perpendicular to the surface.