Abstract

Metropolis Monte Carlo simulations, using semiempirical potentials, are performed to study the structures, stability, and phase transitions of layers of CO and N 2 molecules physisorbed on the MgO(100) surface as well as a monolayer of N 2 physisorbed on NaCl(001) surface. In agreement with experiments [35-37], our simulations show that the c (4 x 2) structure of CO on the MgO(001) surface is the most stable structure below 41 K. The unit cell contains three CO molecules; two bridging molecules tilted in opposite directions by the same polar angle of 31 ̕with respect to the surface normal and a third molecule perpendicular to the surface. At 41 K the c (4 x 2) phase undergoes a transition into a less dense disordered phase accompanied by the desorption of some molecules. The density of this disordered phase is the same as for the p (3 x 2) phase. A model to compare the stability of the c (4 x 2) and the p (3 x 2) phases is constructed and suggests that at sufficiently high pressures and temperatures the p (3 x 2) phase is more stable than the c (4 x 2) phase as found by Panella et al . [35]. We propose that a sequence of transitions to a set of (n x 2) structure with ever decreasing density is possible under suitable conditions of temperature and pressure. This sequence of transitions is an example of the devil's staircase phenomenon as has been suggested by LEED experiments. Nitrogen molecules adsorbed on MgO(001) were studied and were also found to form a sequence of structures. A ([Special characters omitted.] ) R 33.7 ̕structure is found to form the most stable phase at low temperatures (below 20 K). This structure undergoes a phase transition into a less dense phase at 20 K and in all likelihood evolves into the less dense ([Special characters omitted.] ) R 36.9 ̕structure. The ([Special characters omitted.] )R36.9 ̕structure is stable up to 25 K. These results are in agreement with HAS results. This system might also prove to be an example of the devil's staircase phenomena. A monolayer of nitrogen molecules adsorbed on an NaCl(001) surface are used to test the validity of the theory of critical phenomena and its applicability to molecular systems. The simulations predict that this system undergoes a continuous order-disorder transition near 25 K with a logarithmically divergent heat capacity. The values of the critical exponents for the order parameter and susceptibility are calculated and found to deviate significantly from the Ising values but still satisfy Rushbrooke's scaling law. This behaviour is typical of the universality class containing the XY model with cubic anisotropy where the critical exponents are functions of the anisotropy and are thus "nonuniversal"