| Abstract
| - Phonons in the α-quaterthiophene (4T) and α-sexithiophene (6T) polymorph phases are investigated usingthe direct method combined with density functional theory (DFT)-based total energy calculations. Thesimulation of inelastic neutron scattering spectra (INS) on the LT and HT polymorph phases of 4T and 6Tenable the corresponding spectral signatures of these materials to be identified. In particular, there are twofingerprints: (i) the low-frequency vibrational modes (frequencies lower than 200 cm-1) and (ii) the vibrationalmodes in the 600−900 cm-1 frequency range. The good agreement with the INS experimental data allows usto assign unambiguously the origin of all features (first-order and high-order processes) of these spectra andto predict that the LT phase is the phase measured experimentally both on the 4T and 6T materials. Moreover,the broad background in the 600−1400 cm-1 frequency range and the well-defined features which appeararound 940 cm-1 in the calculated INS spectra of 4T/HT and 6T/HT are assigned to multiphonon contributions.This multiphonon contribution at 940 cm-1, which is absent in the 4T/LT and 6T/LT INS spectra, alsoconstitutes a fingerprint of the HT phases. Finally, the calculated dispersion curves of the two polymorphphases of 4T and 6T are given.
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