Neutron inelastic scattering spectrum technique

The dispersion relation contains the most important information concerning vibration normal modes in a crystal. Lattice vibrations can be measured experimentally by means of classical vibration spectroscopic techniques (infrared and Raman) or neutron inelastic scattering. However, only the latter technique allows one to measure the full spectrum in a range of k vectors, whereas with infrared and Raman spectroscopy, only lattice vibrations at T can be detected. This limitation for measuring phonon dispersions is serious, becuase neutron scattering experiments are demanding. 

As Stated in Sec. 1II-6 (aquo complexes), the inelastic neutron scattering (INS) technique is very effective in locating hydrogen vibrations. White and Wright found two hydrogen vibrations at 608 and 312 cm in the INS spectrum of Mn3H3(CO)i2- However, the nature of these vibrations is not clear. 

The two techniques most commonly used to observe vibrational spectra are infrared (IR) and Raman spectroscopy, although other techniques such as neutron scattering [8] can also be employed. Methods for obtaining IR and Raman data are considered in section 4.3, and comprehensive reviews have been given elsewhere [9, 10]. The IR spectrum arises from the absorption of radiation the frequency of which is resonant with a vibrational transition, while the Raman effect results from inelastic scattering of photons to leave a molecule or crystal in a vibrationally excited state. The shift in frequency of the scattered photon corresponds to the frequency of the normal mode that has been excited. 

To obtain further insight into the meaning of the inelastic neutron spectra, it is necessary to have specific theoretical models with which to compare the experimental results. In the harmonic approximation it is possible to calculate the incoherent inelastic neutron spectrum i.e., the neutron scattering cross section for the absorption or emission of a specific number of phonons can be obtained with the exact formulation of Zemach and Glauber.481 A full multiphonon inelastic spectrum can be evaluated by use of Fourier transform techniques.482 The availability of the normal-mode analysis for the BPTI136 has made possible detailed one-phonon calculations483 for this system the one-phonon spectrum arises from transitions between adjacent vibrational levels and is the dominant contribution to the scattering at low frequencies for typical experimental conditions.483 The calculated one-phonon neutron en-... 

As already anticipated, a complementary experimental technique for deriving information on the dynamics (frequencies and vibrational amplitudes) of polymers or of materials in general is the use of inelastic neutron-scattering techniques (INS). After a long development time, during which experiments were difficult and provided limited information, the instruments in a few specialized centers recently began to provide detailed data covering the whole spectrum. Thus, we predict a renaissance of INS techniques for the studies of molecular and lattice dynamics. 

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