Inelastic Neutron Scattering: A Tool in Molecular Vibrational Spectroscopy and a Test of ab Initio Methods

Abstract

Inelastic neutron scattering (INS) as discussed in this article is a technique for studies of the atomic vibrations of solids at low temperature. Unique features of this method are the absence of selection rules and the predominance of scattering due to the motion of hydrogen (but not deuterium) atoms. The scattering from hydrogen is given by a sum of intensities from individual atoms and is easily calculated from a normal mode treatment. When hydrogen is absent, the scattering is usually dominated by coherent contributions involving the motions of pairs of atoms. In either case, ab initio methods can be tested by comparison of the calculated INS spectral intensities as well as the frequencies of molecular vibrations with the observed spectra. For molecular solids, these spectra may be interpreted in molecular terms. In cases where intermolecular interactions are important, these must be included explicitly. Hydrogen-bonded systems are of greatest interest in this respect. Selective deuteration of hydrogen-bonded systems can be used to provide detailed tests of the potential surfaces. At some level of resolution, the effects of intermolecular interactions are revealed even for hydrocarbons.