AUTOIONIZATION OF EXCITED FRENKEL STATES IN THE ANTHRACENE CRYSTAL

Autoionization of Frenkel excitons is re-examined as a potential sourc e of the anthracene photocurrent at photon energies higher than 4.4 eV . A crude estimate of the rate constant suggests autoionization as a f easible depletion channel, the resultant photocurrent quantum yield be ing in reasonable agreement with experiment. Bounds for the thermaliza tion radius of charge carriers produced by autoionization are estimate d. The estimates, based on a simple model of charge carrier transport and scattering, are found to agree, as to the order of magnitude, with experimental data. A fit of the experimental photocurrent spectrum in dicates that while in the near-threshold region the photocurrent is do minated by charge pairs produced by direct optical excitation, at high er energies the autoionization mechanism generally prevails.