Observed INS spectrum of ammonium bromide

The observed INS spectrum of NEUBr is given in Fig. 5.2. The spectrum was taken on the indirect geometry low-bandpass spectrometer TOSCA at ISIS ( 3.4.2.2.2). The spectrum consists of a series of features across the whole spectrum, which sits on a gently rising background. However, bands appear between 500 and 1400 cm, where none were expected and there are no bands about 3000 cm, where the stretches 

This region covers up to ca 300 cm in energy transfer, see Fig. 5.2(c). There are two features, an ill-shaped band at about 80 cm and a finely structured band centred at 160 cm . These bands appear reasonably close to the values suggested from the likely spectrum. The acoustic modes were expected at 130 cm (for ND4CI) but observed at 80 cm for NH4Br. 

We can ascribe these frequency differences to the effect of the different reduced masses, fi, and force constants, / From considerations of simple harmonic motion [3]. 

The optic modes were expected to start about 160 and end about 200 cm, an average of 180 cm. In Fig. 5.2 it starts dramatically, with a very clear van Hove singularity ( 2.6.2.2), at 140 cm and stops, almost as cleanly with a second singularity at 180 cm, an average of 160 cm. Here the ions are moving in antiphase and the reduced mass // is. 

Thus the difference in forces between the chloride and bromide is as important as the changed masses. The intensity in the LO branches extends the optic band out to about 220 cm .  

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