Twisting acoustic mode

Perylene The two different crystalline phases, a perylene and perylene (see Chap. 2, Fig. 2.12) differ strongly also in their dynamic properties a perylene has four molecules or two dimers per unit ceU. From this, 24 internal modes result, 21 optical and three acoustic. They have also been observed and identified by inelastic neutron diffraction [17] and by Raman scattering [18]. Their spectrum has a width of about 4 THz, again similar to the cases of naphthalene and anthracene. For a perylene, the model treated in Sect. 5.6 again yields satisfactory theoretical dispersion relations. The low-energy internal modes are torsional (twisting) and butterfly modes. Their spectrum overlaps with that of the external modes. 

For the phase differences 5 0, the chain vibrations of acoustic branches are associated with the accordion mode (Vj) and chain-twisting mode (Vg) [see Fig. III.5] and the vibrational frequencies are proportional to 5. Accordion vibrations of n-paraffin molecules were observed as Raman lines by Mizushima and Shimanouchi (1949), Schaufele and Shimanouchi (1967). On the other hand, for the phase differences 5 6, the chain vibrations of acoustic branches are associated with the chainbending modes (Vj and Vg) [see Fig. III.5] and the vibrational frequencies are proportional to [5 — 0] [Miyazawa (1967)]. However, for polymer... 

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