Structural characterization of diatomic fluids by diffraction studies

The study of spatial features in diatomic liquids and gases is reviewed with a specific emphasis on experimental measurements using neutron and X-ray diffraction. The formalism for defining the pair-correlation function is described and related to the liquid structure factor which can be extracted from the diffraction measurements. Different molecular systems can be classified in terms of the intermolecular potential which depends on various factors related to geometrical conformation, electron density distribution and anisotropy. The increased complexity over monoatomic systems gives rise to new phenomena associated with the orientational correlation of molecular axes. The techniques of neutron and X-ray diffraction are critically reviewed and a full list of published work (plus some unpublished results) is presented. The experimental measurements are analysed and compared with the results of computer simulations using model potentials. It is noted that several systems (e.g. N₂, O₂, F₂, CO) have a relatively simple behaviour but others (e.g. Cl₂, Br₂, I₂) are only just on the point of being properly understood, while the hydrogen-bonded liquids (e.g. HF, HCl) still pose many problems that currently remain unresolved. The review ends with a brief consideration of likely developments in the foreseeable future.