Local structure of liquid-vapour interfaces

The structure of a simple liquid may be characterized in terms of ground state clusters of small numbers of atoms of that same liquid. Here we use this sensitive structural probe to consider the effect of a liquid-vapour interface upon the liquid structure. At higher temperatures (above around half the critical temperature) we find that the predominant effect of the interface is to reduce the local density, which significantly suppresses the local cluster populations. At lower temperatures, however, pronounced interfacial layering is found. This appears to be connected with significant orientational ordering of clusters based on three- and five-membered rings, with the rings aligning perpendicular and parallel to the interface respectively. At all temperatures, we find that the population of fivefold symmetric structures is suppressed, rather than enhanced, close to the interface.