Order–disorder transitions in quasi-two-dimensional argon solvent clusters

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Abstract
Structures and order–disorder transitions of the rare-gas solvent clusters carbazole⋅Arn were studied for n=1–7 by molecular dynamics and Monte Carlo simulation methods. This study was motivated by the investigation of these clusters by size-specific electronic spectroscopy. Cluster structures corresponding to absolute and local energy minima were obtained by molecular-dynamics slow cooling and annealing. For n=1–3, single minimum-energy structures of Cs symmetry were found, with all n atoms on the same side of the molecular substrate. For n=4–7, the number of energetically low-lying monolayer isomers rises very rapidly with n. Orientational ordering of the adcluster by the substrate is unimportant for n≥4. The isomers for n≥4 differ by (i) rotational/translational displacements of the cluster relative to the substrate, (ii) promotion of atoms to the second solvent layer, and (iii) adsorption of atoms on the second side of the substrate. Order–disorder transitions of the monolayer solvent clusters were studied by MC methods as a function of temperature. Four different quasi-two-dimensional order–disorder transitions were found: the racemization and surface-decoupling transitions reflect the loss of cluster–substrate orientational correlation, while the cluster rigid–fluxional and melting transitions derive from the loss of intracluster bond-orientational order, and increase in bond-length fluctuations. Analogies to order–disorder phase transitions in two-dimensional rare-gas films are discussed.

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