Vibrational properties of a series of individual pi-conjugated alpha-coupled oligothiophenes alpha-nT have been investigated by resonance raman scattering (RRS) and infrared absorption spectroscopies. The experimental frequencies obtained on crystalline evaporated alpha-nT (n = 4, 6, and 8 thiophene rings) thin films are compared with those calculated using a symmetrized dynamical matrix model. Special attention is given to the 1300-1600 cm(-1) region where the C-alpha=C-beta and C-beta-C-beta stretching vibrations are active. Also, vibrational spectra of doped alpha-sexithiophene in the solid state are reported where we observe remarkable changes when using different excitation wavelengths. It appears that upon doping, the alpha-sexithiophene polymerizes and the consequent doped poly(sexithiophene) presents a distorted conformation around the inter-ring single bonds. In addition to a complete assignment of the various vibrational modes of each alpha-nT molecule, our results on well-defined model oligomers shed some light on the controversial parent polythiophene and poly(3-alkylthiophenes).