SIZE DEPENDENCE OF THE ORIENTATIONAL DISORDER AND MELTING TRANSITIONS OF FINITE N-2 ISLANDS DEPOSITED ON GRAPHITE

The orientational order-disorder and melting transition temperatures for quasi two-dimensional islands of N2 molecules deposited on graphite are calculated for sizes 1 less-than-or-equal-to N less-than-or-equal-to 256 molecules, using a Monte Carlo procedure with free surface boundary conditions. These transition temperatures are strong functions of N, and the values for 256 molecules are close to those measured experimentally for fractional macroscopic monolayers. Calculated results show that the two sublattice herringbone orientational structure persists up to the orientational transition temperature, above which hindered planar rotation occurs. The registered square-root 3 X square-root 3 mass center structure exists until melting, where the islands dissociate. These features are consistent with experiment. Below melting, the root mean square fluctuations of the mass centers away from their square-root 3 X square-root 3 equilibrium values are substantially larger than for bulk N2 or for small three-dimensional rare gas clusters. The fluctuations are also substantially larger for edge molecules than for those in the center.