Abstract

The adsorption of CO on NaCl(100) at monolayer, submonolayer and multilayer coverages, and on LiF(100) at monolayer coverage were studied. The Metropolis Monte Carlo method was used to study the structures of the overlayers and to characterize the phase transitions. The simulations were performed in the temperature range from 1K to 75K. At low temperature and monolayer coverage, both of these systems form ordered phases that become disordered as the temperature is increased. For the CO/NaCl system the estimated transition temperature is between 32K and 35K whereas for CO/LiF a temperature between 40K to 45K is estimated. Below the transition temperatures, both systems have an ordered p(2x1) type structure due to correlated azimuthal orientations. The ordered phases have two molecules per unit cell connected by a glide plane. In the CO/NaCl case the projections of the molecular axes upon the surface are antiparallel. We find that in monolayer of CO/NaCl the molecules are tilted by about 27Å whereas larger tilt angle (k V 44Å) is observed in a monolayer CO/LiF. At high temperatures, both systems undergo a phase transition to an azimuthally disordered p(1x1) phase, i.e. one with no preferred orientation in the surface plane. Yet on average, the molecular axes remain tilted with the surface normal. Evidence for a continuous order-disorder phase transition is found in the form of a characteristic divergence of the heat capacity and susceptibility within the estimated transition temperature range. Coverages of less than a monolayer of the CO/NaCl system have also been studied. The CO molecules are found to aggregate and form islands with an ordered structure in the middle of the islands. Apparently, these islands also undergo an order-disorder transition but at lower temperatures. At high temperatures, gas-solid coexistence in two dimensions is also observed. Simulations of adsorption of multilayer systems are found to destabilize the p(2x1) phase of the bottom layer in favour of a p(1x1) structure with the upper layers adopting the correct orientational ordering of the bulk solid Ì-CO. Nonetheless, the molecules fail to form a correct head-to-tail ordering as in the bulk structure. Preliminary results show that the ability to adopt the correct bulk structure depends upon the number of the layers present in the system. The more layers the system has the better chance the system has to adopt the correct head-to-tail ordering.