Photoconductivity of crystalline anthracene induced by tunable dye lasers

We present the results of an experimental study of the bulk photoconductivity of crystalline anthracene induced by high-power (I1022-1024-photon cm-2 sec-1) tunable dye lasers in the wavelength region λ=6943-4150 . The kinetic data for the dependence of the charge carrier density m on the excitation intensity I fall into three regions: (I) In the excitation region λ=6943-6180 a mI3 dependence has been observed originating from photoionization of two-photon-excited singlet states. (II) In the range λ=6120-4600 a complex, superlinear pattern was observed, exhibiting a mI2 dependence at low I and mI4 dependence at high I. In this range, where excitation occurs via two-photon absorption, three charge-carrier-generation processes involving autoionization of metastable excitons, exciton photoionization, and exciton-exciton collision ionization are operating. Detailed information was obtained regarding the energy dependence of the generation coefficients for these processes. (III) In the excitation range λ=4600-4400 a mI2 dependence was observed and interpreted in terms of photoionization of one-photon-excited singlet states. From the generation coefficients in regions I and III and from the mI3 generation coefficient in region II, detailed information has been obtained regarding the energy dependence of the cross section for exciton photoionization. From the mI4 generation coefficient in region II we have obtained the rate coefficient (4 ± 2) × 10-12 cm3 sec-1 for the exciton collision ionization while the autoionization efficiency of metastable excitons produced by exciton collisions is (7 ± 4) × 10-5. The two-photon-induced mI2 generation coefficients in region II, normalized by the two-photon-absorption cross sections, result in a detailed energy dependence of the autoionization efficiency η(E). The direct onset to the conduction band is Eg=4.08±0.04 eV. η(E) yields a reasonable measure for the weighted density of states for interband transitions, providing experimental evidence for narrow (∼ 1000- cm-1) vibronic components of the valence and conduction band, which exhibit vibrational structure near the threshold.