Inelastic neutron scattering INS —a spectroscopic technique

The most common methods for studying molecular vibrations are the well established optical techniques of infrared and Raman spectroscopy. It is through a direct comparison with these techniques that the advantages of INS can be most readily grasped. 

Optical (i.e. infrared and Raman) techniques are, generally, most sensitive to vibrations involving the heavier atoms, because of the number of their electrons. The neutron incoherent scattering cross section of hydrogen is uniquely high, and makes it about ten times more visible than any other atom. 

All vibrations are active in INS and, in principle, measurable. This stems from the mass of the neutron ca 1 unified atomic mass imit). When scattered the neutron transfers momentum to the atom and INS measurements are not limited to observation at the Brillouin zone centre, as are photon techniques. The measured INS intensities are, inter alia, proportional to the concentration of the elements in the sample. 

Neutrons penetrate deeply, of the order of millimetres, into typical samples and pass readily through the walls of containment vessels, generally aluminium or steel. INS results are thus naturally weighted to the measurement of bulk properties. 

INS spectrometers cover the whole molecular vibrational range of interest (16-4000 em, see Fig. 1.1). The lower energy range (below 400 cm ) is readily accessible, a region that is more difficult experimentally for infrared and Raman spectroscopies. With modem instrumentation, the quality of INS spectra approaches that of infrared and Raman spectra obtained from the same system under the same conditions. 

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