Electron Correlation Functions in Liquids from Scattering Data

Recent work on the theory of liquid metals has involved correlation functions for the ion–electron density and the electron–electron density. The experimental determination of these functions is discussed for a general homonuclear fluid. It is shown that the electronic correlation functions may be extracted, in principle, by combining X-ray, neutron, and electron scattering data, though the smallness of the differences between the normalized data makes this difficult to do at present. After reviewing the published scattering data, we conclude that the most useful procedure is to compare scattered intensities at a significant reference point, namely the principal maximum of the liquid structure factor. X-ray and neutron data are presented for liquified rare gases, molecular liquids, and liquid metals, and their averages considered. Systematic differences are found between these groups, which prompt the conclusion that electrons in molecular liquids are delocalized by chemical bonding, but that the conduction electrons in a liquid metal have a higher degree of short range order than the nuclei. In this event an electron liquid, rather than an electron gas, exists in liquid metals.