EXCITON ELECTROABSORPTION AT NONZERO WAVE VECTOR

Based on a previously proposed two-dimensional model of a polyacene cr ystal, the contributions to the electro-absorption signal from differe nt points of the exciton Brillouin zone are calculated. They are shown to differ substantially both in amplitude and in shape. This makes el ectro-absorption spectra very sensitive to vibronic coupling. On the o ne hand, the new factor is a potential complication in theoretical int erpretations. On the other hand, it favours electro-absorption spectro scopy as a promising experimental tool to study vibronic effects.