OBSERVATION OF SINGLET EXCITON PHOTOIONIZATION IN ANTHRACENE SINGLE-CRYSTAL AT 2.95 EV

Photo-carrier generation with 420 nm light (hv=2.95 eV) in an anthracene single crystal has been studied. The main experimental results are the following: (1) The photocurrent waveforms correspond to the penetration depth of the exciting light. (2) The number of carriers generated is proportional to Iex2 and becomes proportional to Iex 3 4 under strong excitation. (3) The photocurrent has a temperature dependence different from that of the absorption coefficient of the ground state. These observations indicate that charge carriers are generated through photoionization of a singlet exciton, i.e. by sequential two-step excitation. With an applied electric field of 1.5×104 V cm-1, the cross section of the singlet exciton photoionization (with λ=420 nm) is 7×10-19 cm2. The Onsager length r0, determined from the temperature dependence of the photocurrent, is 36 Å. © 1990.