We have investigated the experimental terahertz phonon modes in a model oligothiophene, the alpha-quaterthiophene (LT phase), by using infrared and inelastic neutron scattering spectroscopy in the 10-50 cm(-1) (0.3-1.5 THz) domain. The ability of density functional theory to reproduce the terahertz infrared phonon modes is also reported. In agreement with the calculation, the experimental infrared spectrum at 10 K is dominated by two phonon modes centered at 32 and 46 cm-(1) respectively assigned to an antiphasis translation mode and a libration of rings. (C) 2007 Published by Elsevier B.V.