Radiation scattering data on liquid metals

Abstract

The scattering of radiation by liquid metals provides data on both the microscopic and the macroscopic properties of the metal. The macroscopic properties are obtained as a limit in which the momentum transferred by the radiation to the liquid is vanishingly small, while the microscopic structure and dynamical behaviour are obtained from data at higher momenta. An outline of the theory of radiation scattering will be given which emphasizes the relation between neutron and x-ray results. The possibility of studying electron shell movements by measuring the ratio of x-ray to neutron intensities is discussed. In general there are two types of experiment: those in which the intensity is measured as a function of the momentum transfer only (S(Q)), and those in which the intensity is measured as a function of both energy and momentum transfer (S(Q, θ)). The former experiments are used to obtain information on atomic positions while the latter experiments (coupled with the former) are used to derive dynamical information. Both kinds of experiment will be discussed and some methods of interpreting (S(Q)) and S(Q, θ)) will be described. The cooperative 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, θ). Several recent experiments of this kind on liquid lead will be discussed, particularly those in which the temperature variation was studied. In addition, information on the velocity correlation function for the metal atoms may be obtained. The spectral density of the velocity correlation will be considered and compared to predictions based on the cooperative mode picture. It is concluded that while much information has been obtained from these experiments there is a more detailed and extensive work to be done with both neutrons and x-rays.