Theoretical interpretation of the electroabsorption spectra of polyacene crystals. II. Charge-transfer states

In recent articles, a two-dimensional analog of the Merrifield model of the coupling between the Frenkel and charge transfer (CT) excitations of a mol ecular crystal was successfully applied for the calculation of the electroa bsorption (EA) spectra of polyacene crystals. Presently, the results of tho se calculations are rationalized in terms of a simplified two-state model, designed specifically for interpretational purposes. Particular attention i s given to the CT manifold where symmetry considerations rule out the exist ence of eigenstates endowed with nonzero dipole moment, seemingly contradic ting the existing experimental evidence of second-derivative EA signals. Th e apparent contradiction is resolved by analyzing EA contributions from pai rs of zero-field eigenstates coupled by the modulating field. The new appro ach provides a bridge to the conventional interpretation rooted in the loca lized model of CT excitations, and allows to determine the limits of validi ty of the traditional analysis of EA spectra. (C) 1999 American Institute o f Physics.