The polarized infrared absorption and Raman spectra on the bc plane of a single crystal of alpha-sexithienyl (alpha-T6) have been recorded in the range from 50 up to 3500 cm(-1). The intermolecular interactions give rise to the Davydov splitting of the ungerade phonon energy levels originated by the molecular pyramidalization along with the CCH bending modes. A mu-Raman investigation on the crystal surface allows identification of the perturbed local packing of the molecules, which are spectroscopically crystal defects. The selective resonant enhancement of the scattering intensity of some Raman modes, obtained by exciting at lambda(ex) = 632.8 nm, indicates that these defects are associated with the lowering of the electronic excited states due to the increased pi-orbital overlapping. Moreover, the resonant enhancement allows detection of some of the gerade intramolecular phonons not observed in the bulk. The comparison between the far-infrared absorption spectra recorded at room temperature and at 80 K evidences three lattice phonons. The spectral analysis, based on a simulation, which uses a scaled ab initio molecular force field, well interprets the experimental data assigning all the internal phonons and elucidates some of the effects of the intermolecular interactions. The calculations are performed at the HF/6-31 G* level of the theory to obtain the harmonic frequencies and the dipole moment and polarizability tensor derivatives of the alpha-T6 molecule. Both polarized infrared and Raman spectra simulations are carried out calculating the optical response of a unit cell of the alpha-T6 single crystal belonging to the C-2h factor group in the monoclinic P2(1)/n space group. (C) 2000 American Institute of Physics. [S0021-9606(00)70513-7].