Exciton induced photoemission in anthracene

Abstract

The double quantum external photoemission of electrons into vacuum which occurs when single crystals of anthracene are illuminated with light of photon energy greater than 4 eV is shown to arise from the interaction of a Frenkel exciton (3.1 eV energy) with a bulk photo-generated electron to yield an emitted electron. This is demonstrated by a double light source experiment in which a second light beam of a 3.1 eV energy (393 nm), which in first order only produces Frenkel excitons, is simultaneously illuminated on the crystal surface which is photoemitting electrons by primary excitation with light in the energy range 4-5 eV. The effect of the former is to greatly enhance the photoemission of the latter by an amount which depends linearly on the number of added excitons. The observed double quantum photoemission is qualitatively similar to that previously reported for anthracene and which has been interpreted as being due to charge-transfer exciton annihilation. There is no need to postulate the participation of charge-transfer excitons to explain the double quantum photoemission of anthracene for excitation energies greater than 4 eV in our experiments. © 1971.