Inelastic neutron scattering is the process by which neutrons collide with atomic nuclei and either gain or lose kinetic energy. This process is shown schematically below.

An Inelastic Neutron Scattering Experiment

The incident neutron has energy E_o. It strikes the sample target and after interaction has a final energy E_f. The change in energy is due to energy transfer to or from the sample. In our experiments the sample is at low temperature (< 20K) so the incident neutron loses energy. The sample gains energy. We are primarily interested in those cases where the excitation of the sample is due to vibrational motions. This means that this method is similar in some respects to infrared absorption or Raman scattering.

Inelastic neutron scattering is conceptually a lot like Raman scattering. There are, however, significant differences. The major one is that photons (electromagnetic radiation) interacts primarily with the electrons of a sample. Neutrons interact with the nuclei. The strength of the interaction is very dependent on the particular nuclide involved. For example, hydrogen (H) atoms scatter much more strongly than deuterium (D) atoms. Another difference is that the wavelength of the electromagnetic radiation used for Raman experiments in the visible or near UV region has wavelengths of hundreds of nanometers (thousands of Ångstroms). Neutrons with energies corresponding to room temperature have wavelengths slightly less than 2 Å. The neutrons used for our experiments have wavelengths of 0.1 to 2 Å. This means that the size of the neutron is comparable to or less than the spacing between atoms. This results in their being no "selection rules" as is the case for optical spectroscopy.