Charge-transfer transitions in crystalline anthracene and their role in photoconductivity

Electric-field modulated absorption spectra of polycrystalline anthracene layers delineate existence of a series of five charge-transfer bands that can be assigned to transitions within the ab crystal plane. The energy versus distance relationship is coulombic (e = 3.2) yielding an optical band gap E_g^opt = 4.4±0.05 eV. Absorption coefficients are about one order of magnitude lower than calculated by Bounds et al. and indicate a coupling constant A ～ 0.15 for interaction between CT and Frenkel exciton states. Previous data for intrinsic free-carrier production, in particular the energy dependence of the “thermalization” distance, can be consistently interpreted in terms of dissociation of CT pairs if the assumption is made that the vibrational CT energy (0.3 eV) can fully or in part be used for additional separation of the electron-hole pair. The adiabatic (electrical) band gap is E_g^e1 = 4.1±1 eV.