On the origin of low quantum yields for photoconductivity in anthracene

Recent experiments have shown that immediate recombination of electron-hole pairs created by high energy radiation in anthracene crystals is a very probable process. The high recombination probability arises because the mean free path of electrons and holes in anthracene is small compared with the distance from a unit charge at which the Coulomb potential energy is equal to the thermal energy. In view of these results, the question which immediately arises is how important is immediate recombination in low energy band-to-band transitions in anthracene. A number of experiments reported recently have generally given a quantum efficiency of the order of 10(-4); since the experiments involved a variety of excited states, it appeared that immediate recombination was solely responsible for the low quantum efficiency. On the basis of this hypothesis, absorption of photons by singlet excitons in anthracene was looked for. The cross section for absorption predicted by the hypothesis, based on the measured cross section for exciton photoionization, is 10(-15) cm(2); experimentally, it was found to be less than 10(-17) cm(2) at 5300 A. Therefore, it appears that the low quantum yield observed for anthracene is not solely the result of a high probability for immediate recombination but must also involve low probabilities for autoionization of highly excited but bound states.