Photo-Ionization of Two-Photon-Excited Singlet Excitons in Anthracene

The results of an experimental study of the photoconductivity induced by a giant-pulse ruby laser in crystalline anthracene lead to the following conclusions: (1) Up to laser intensities of 5×${10}^{25}$ photons ${\mathrm{cm}}^{-2\mathrm{}}$ ${\sec }^{-1\mathrm{}}$, charge carriers are generated by a three-photon process. (2) The dominant mechanism for the production of charge carriers involves the photo-ionization of singlet-exciton states. (3) A cross section of ${10}^{-19\mathrm{}}$ ${\mathrm{cm}}^2$ at $\lambda =6940\mathrm{}$ Å was determined for the photo-ionization of singlet-exciton states. (4) A bimolecular rate constant of (5±3)×${10}^{-7\mathrm{}}$ ${\mathrm{cm}}^3$ ${\sec }^{-1\mathrm{}}$ for charge-carrier recombination in crystalline anthracene was directly determined.