Exciton Electro-Absorption at Non-Zero Wave Vector

Based on a previously proposed two-dimensional model of a polyacene crystal, the contributions to the electro-absorption signal from different points of the exciton Brillouin zone are calculated. They are shown to differ substantially both in amplitude and in shape. This makes electro-absorption spectra very sensitive to vibronic coupling. On the one hand, the new factor is a potential complication in theoretical interpretations. On the other hand, it favours electro-absorption spectroscopy as a promising experimental tool to study vibronic effects.