Intrinsic Photoconductivity in Naphthalene Single Crystals

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Abstract
For naphthalene single crystals, we observe charge carrier generation which exhibits quadratic intensity dependence for photon energies from 3.8 to 5 eV. Our experiments appear to rule out involvement of triplet excitons, and we attribute ionization to the mutual annihilation of two singlet excitons. The room temperature rate constant for singlet–singlet annihilation is calculated to be γ ≤ 1.7 × 10−9cm3sec−1 and that for carrier generation is β = 2.3 × 10−14cm3sec−1. These values are compared with those for anthracene where the latter values are revised based on a recent estimate of the intrinsic anthracene singlet exciton lifetime. For photon energies above 5 eV, we observe increased carrier yield and decreased intensity dependence exponent and tentatively ascribe these results to onset of single-photon ionization at about 5 eV.