Vibrational Spectroscopy with Neutrons: A Review of New Directions

Inelastic neutron scattering spectroscopy (INS) has enabled vibrational spectra to be measured for over 50 years. Most studies have used a type of spectrometer that is straightforward to build and use and that provides spectra that are not dissimilar to infrared and Raman spectra. In this Focal Point Review Article we show the advantages of a type of neutron vibrational spectrometer that has been largely unknown to the spectroscopy community. These instruments are able to access regions of low momentum transfer at relatively large energy transfer. This means that the C-H, N-H, and O-H stretch regions can be exploited by INS spectroscopy for the first time. The instruments generally have very large detector area, which means that they are significantly more sensitive than the more commonly used instruments. They also allow the energy transfer as a function of momentum transfer to be examined. After briefly outlining the basics of INS spectroscopy, we describe the operational principles of the instruments and show how flux and resolution can be traded. We then review how the advantages of the instruments can be used to gain understanding of molecular systems in areas as diverse as hydrogen storage, hydrogen bonding, and fullerenes. The instruments are starting to have a significant impact in studies of catalysts and this is illustrated with recent studies of hydrogen on fuel cell catalysts, methyl chloride synthesis, the deactivation of methane reforming catalysts, and a model carbon monoxide oxidation catalyst.