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 m∝I³ 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 m∝I² dependence at low I and m∝I⁴ 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 m∝I² 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 m∝I³ generation coefficient in region II, detailed information has been obtained regarding the energy dependence of the cross section for exciton photoionization. From the m∝I⁴ 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 m∝I² 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.