Radiation scattering studies of the structure and transport properties of liquids

Abstract

The use of radiation scattering techniques to study problems in liquid state physics has several advantages. These include the detailed nature of the information which can be obtained on structural and dynamical properties, the definiteness of the interpretation which may be given to the data and the exceedingly wide range in types of sample and experimental circumstances which can be covered. To deploy these advantages to the full requires powerful sources and special experimental methods which are still under development. A review is given of the techniques and theory of radiation scattering work emphasizing the relation between neutron and X-ray methods. The possibility of studying electron shell movements by measuring the ratio of X-ray to neutron intensities is discussed.

Broadly, the kinds of experiment fall into two classes: (a) those in which the scattered intensity is measured as a function of the momentum transferred (ħQ) by the radiation to the specimen, giving the function S(Q), and (b) those in which the intensity is measured as a function of both the momentum transferred and the energy transferred (ħω) by the radiation to the specimen, giving the function S(Q, ω). The former are used to obtain information on atomic positions while the latter experiments (coupled with the former) are used to derive dynamical information. Both kinds are discussed and some methods of interpreting S(Q) and S(Q, ω) are described.

Co-operative modes of motion in the system, particularly for wavelengths of the order of the spacing between atoms, may be observed as peaks in S(Q, ω). The relationship between this kind of data and general transport coefficients is discussed. In addition, the spectral density of the velocity correlation function for atomic motions may be obtained from S(Q, ω), and this relationship is outlined. This paper covers only the theoretical background to the field.