Low momentum transfer spectra

Direct geometry spectrometers have better access to low Q regimes at higher energy transfers than low-bandpass spectrometers and this is a considerable advantage. At low Q the phonon wing has not yet developed and its contaminating effects are much reduced. 

15 The INS spectrum of Rb2[PtH6] obtained at low momentum transfer (MARI, ISIS), showing the effective suppression of the phonon wing contributions. Reproduced from [20] with permission from the American Chemical Society. 

Unfortunately at low Q the strength of the observed bands is also underdeveloped and rather weak, long counting times are needed to obtain the good statistics needed to carry the analysis further. An example of the quality that is available from low Q data is given in Fig. 5.15. The problems that can arise from a too strongly scattering sample, measured at low Q, have already been discussed ( 3.5.1). 

In this chapter we have presented and discussed the use of the basic tools for spectral analysis. However, it should always be remembered that synthetic methods are more powerful, more likely to achieve success and, probably, quicker. Analytical methods should only be used to treat spectra that are either, particularly straightforward, or, paradoxically, particularly difficult. The first category represents simple problems that can be dealt with more efficiently by a rapid analysis than by a drawn-out ab initio calculation. The second category represents a failing synthetic approach and under these circumstances analysis offers, possibly, the only way forward. 

Woessner B.S. Snowden, Jr. (1967). J. Chem. Phys., 47, 378-381. Proton spin-lattice relaxation temperature dependence in ammonium bromide. 

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