The primary purpose of this work is to describe the use of neutron scattering in the study of molecular solids and liquids. The development of the material is such that it is accessible not only to those already working with neutrons as a tool in molecular research, but also to optical spectroscopists and those in related fields who may be interested in the results produced and in the techniques themselves.
The quantitative study of the structure and properties of matter necessarily involves detailed considerations of the forces acting between atoms and molecules. The determination of frequencies of vibration between atoms in a molecule (internal modes) and between molecules in a solid or liquid (external modes) is a basic problem in molecular spectroscopy, and a knowledge of these frequencies leads directly to information about interatomic force constants and intermolecular potential barrier heights. Such investigations traditionally have been carried out by means of optical measurements such as infrared absorption and Raman scattering. These are highly refined, powerful techniques, and they have produced valuable and extensive data.
Recently, however, the increasing availability of nuclear reactors, with their large flux of low-energy neutrons, has enabled new investigations of molecular motions to be initiated using neutron methods. The technique of neutron scattering has already produced much useful new information about crystal dynamics and atomic motion in simple liquids.
Neutrons of very low energy (thermal or subthermal) are a unique probe for the study of molecular dynamics in solids and liquids, for several reasons. For one things, since the neutron energy is comparable to that of lattice vibrations and molecular rotations, the motions can be observed with relative ease and studies in detail by means of inelastic scattering. Also, since the mass of the neutron is of the same order as the mass of the scattering nuclei, the scattering is particularly sensitive to the structure of the system.
It is interesting to note that for molecular phenomena the relevant energies are of the order of 10-3 to 10-1 eV and the wavelengths are of the order of 10-8 cm, and for this reason neutron scattering is at present the only probe capable of revealing the details of dynamical processes on such short space and time scales. In the case of optical scattering and absorption, the frequency range is essentially the same, but the wavelengths are longer by three or more orders of magnitude. In spite of the quite different interactions involved in the two methods, basically the same dynamic properties can be measured with neutron scattering and infrared absorption. But because each method probes a different wavelength-frequency region, it is to be anticipated that the information derived from the two techniques will more likely be complementary, and will become identical only in special cases.
The most commonly used experimental techniques are the chopper time-of-flight technique and the rotating-crystal time-of-flight technique. Both involve a pulsed beam of monoenergetic neutrons scattered through various angles being measured by time-of-flight. These experimental techniques are discussed in the last chapter.