Inelastic neutron scattering measures the change in the energy of the neutron as it scatters from a sample. This can be used to probe a wide variety of different physical phenomenon: diffusional or hopping motions of atoms, the rotational modes of molecules, sound modes and molecular vibrations, recoil in quantum fluids, magnetic and quantum excitations or even electronic transitions.
Often knowing the atomic structure of a material will be sufficient to understand its nature. But to gain a deeper insight into the underlying physics of say a phase change, it is necessary to also understand the atomic dynamics. The vibrational motion of atoms is, entirely or in part, responsible for a large number of the characteristic properties of a material, such as the specific heat, thermal conductivity, optical and dielectric properties and electrical resistance, but it is also a direct way of understanding the nature of atomic bonding. And with the current interest in smart or functional materials whose properties are often determined by a complex balance or strong coupling between competing phenomena, understanding the atomic and magnetic dynamics is essential.
The most commonly used spectroscopies are the light scattering techniques: Raman and infrared. Compared to neutron scattering they are cheap, readily available and highly sensitive. But they do have certain limitations: They can only measure near the Brillouin zone centre and are only sensitive to certain vibrational modes. The calculation of the scattered intensity is also difficult and prone to error and so information is usually only taken from the positions of the observed modes. However, the simplicity and sensitivity of the techniques means that they are often used to identify or ‘finger print’ compounds something that is rarely done with neutrons. Inelastic neutron scattering is usually used to understand the physics of a system. It is a highly quantitative probe whose results are directly comparable to numerical and analytical calculation. It can be used to understand the nature of a phase transitions or linked directly to thermodynamics quantities, like specific heat or thermal conductivity, or structural properties such as force tensors or bulk and shear moduli. It is still one of the few methods available to measure phonon and magnon dispersion curves. And, due to the unique nature in way that hydrogen scatters neutrons, it is a natural technique for measuring the vibration or diffusion of hydrogen in a material.