Low-lying valence band states and intrinsic photoconductivity in crystalline anthracene and tetracene

The wavelength dependence of the intrinsic photoconductivity of anthracene and tetracene single crystals has been measured with photon energies up to 6.5 eV. Both materials exhibit three peaks or shoulders in the 3.0-6.5-eV range. The photoconductivity spectra are interpreted by comparing them to the kinetic energy distributions of external photoemitted electrons. The energy sequence of the photoconductivity peaks is in good agreement with characteristic energy losses suffered by the externally emitted photoelectrons. The decreases in the photocurrent, as the photon energy is increased, are shown to arise at energies at which transitions from lower-lying valence states set in. When transitions from lower-lying valence states are involved, the energy of the final state may be below the photoconductivity threshold energy and the current decreases. The energy sequences of the valence states are compared to theoretical HMO and extended-Hückel-method calculations. Finally, the absorption spectra of these crystals are related to the photoconductivity action spectra.