Impact of the external modes

First we introduce a more realistic approximation to the phonon spectrum. The phonon spectrum is generally taken to be the sharply structured low-energy part of the observed INS spectrum. In the case of benzene the selected spectral range is between 20 and 150 em. Using 

Following a number of manual iterations in the program to best match the intensities the external modes displacement tensor y ext is extracted, see Fig. 5.6. 

In this specific case the Cartesian frame of the tensor is referred to hydrogen in a C-H bond with its direction along the j axis, the x axis lies perpendicular to y in the benzene plane and z is normal to the plane. The anisotropy is very clear, with the greatest value being six times that of the smallest. 

The tensor of Eq. (5.16) can be compared with the crystallographic results provided they are referred to the same frame and obtained at the same temperature (from perdeuterobenzene at 15 K [18]) 

Cleeu ly the two techniques, diffraction and INS, are in good agreement, given that Eqs. (5.16) and (5.17) are the result of approximations. 

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