Worked example—phonon wings of the bifluoride ion

We remain with our earlier example of the [FHF] ion and since this is centrosymmetric only translational external vibrations will result in the displacement of the hydrogen atom. The dynamics of the low cross section fluorine atoms can be ignored. The ionic mass is 39 amu and the total hydrogen displacements (A) of the internal modes are ( 2.5.3)  

The anisotropic displacement parameters of the hydrogen atom, as determined fi om diffraction studies [10], are  

These represent the total vibrational displacements, internal and external, of the hydrogen atom ( 2.4), so using  

We may extract the external A tensor of the hydrogen. Furthermore, we can determine the external a values that control the distribution of intensities arising from internal vibrations. 

The antisymmetric stretch, at 1370 cm, will be used as the example, and from Eq. (2.61)  

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