X-Ray Scattering from Two-Dimensional Liquids Modulated by a Periodic Host: Theory, Simulation and Experiment

Abstract

A fluid layer on a surface, such as Helium on graphite, or intercalated between the planes of a crystal, such as Rubidium in graphite, provide examples, upon solidification, of incommensurate solids. Despite extensive theoretical and experimental work [1], the structure of these solid phases is still controversial. This is due to the difficulty of extracting the structure from the available experimental evidence [2], the difficulty of doing an accurate theory in the regimes of interest, and the lack of knowledge of essential physical parameters, in particular, the substrate fluid potential and the interaction potential between the atoms or ions of the fluid. The latter quantity is not actually used in most theoretical predictions [3], which use instead effective parameters which would be difficult to relate to the physical parameters even if known, making possible a wide range of theoretical predictions. We present here theoretical, numerical and experimental results for Rb intercalated in graphite in the fluid phase and show that it is possible to extract from the x-ray scattering data the substrate-intercalant potential. We expect to be able to extract as well the intercalate-intercalate interaction potential, making possible molecular dynamics simulations that should reveal the structure of the solid phase as well as permit an interpretation of neutron scattering data on the dynamics of the intercalate. The essential features of the x-ray scattering patterns are exhibited and explained theoretically.