PHOTOEMISSION FROM SMALL ORGANIC CRYSTALS INTO AN AMBIENT NITROGEN ATMOSPHERE (TETRACENE, SCAVENGING, ELECTRON, THERMALIZATION)

A modified Millikan apparatus was constructed and used to measure photoemission from micron-sized charged crystals in relatively dense nitrogen in the presence of a levitating external field. The escape of emitted electrons from the particle was found to depend upon the particle size and state of charge, the electron kinetic energy, the magnitude of the external field, the gas pressure and the gas composition. A theory is presented to describe the probability of escape as a function of all of the above parameters. Epithermal scavenging by electron attaching molecules present in low concentration in the ambient atmosphere is a requisite feature of the theory. The ionization potentials of anthracene, tetracene, and perylene were measured in 1000 Torr nitrogen gas. The results are found to compare favorably with published results for high vacuum photoemission measurements. The double quantum external photoemission effect (DQEPE) of tetracene was studied. The tetracene DQEPE is due to the Auger dissociation of a bimolecular excitonic state consisting of two electrons and one hole. This state is generated by the fusion of a mobile electron with a trapped charge-transfer (CT) exciton. The observed tetracene DQEPE can exhibit various light intensity dependences. This may be explained by invoking a process of electron detrapping by triplet excitons from recombination centers in the crystal and a theory to account for this effect is presented. The existence of a long lived (10('-6) sec) CT exciton in tetracene is indicated by the theory.