Knowledge Bank

Inelastic neutron scattering may be used in studies of internal molecular vibrations in a fashion similar to Raman scattering. It is well known that neutron scattering intensities are often quite different from the corresponding Raman or infra-red intensities. Another unique feature of neutron scattering is the fact that the intensity of an inelastic transition can be calculated directly from an assumed knowledge of the motions of the nuclei without any need for a knowledge of the electronic motions. This knowledge of the nuclear motion can, in turn, be derived from an assumed molecular potential energy function by solving for the normal mode eigenvalues and eigenvectors. The formalism of Zemach and $Glauber1$ has been programmed in a form suitable for calculations of vibrational intensities for large molecules. The potential function of Lifson and $Warshel2$ has been used to generate the normal mode eigenvectors and eigenvalues for polyatomic hydrocarbons. The results are compared to the experimental spectra of Strong and co-$workers3$ on solid, liquid or gaseous samples of ethane, propane, n-butane, isobutane, neopentane and cyclohexane. Calculations of this type are shown to be sufficiently reliable that they are useful in several respects. They may be used as an aid in the assignment of spectra. Defects in the potential function may be detected. Approximate calculations of inelastic neutron cross sections can be tested by comparison with the exact calculations. Finally, neutron scattering spectra can be predicted for interesting and perhaps complex molecules. Each of these applications will be illustrated.