Photoemission from Polycyclic Aromatic Crystals in the Vacuum Ultraviolet Region

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Abstract
The external photoelectric effect of thin films of tetracene, pentacene, violanthrene, quater-rylene and tetrathiotetracene was studied in the vacuum ultraviolet region. The contact potential differences between the emitters, organic crystals, and the collector (Aquadaq) were determined from the saturation points of the current-voltage characteristics. It was observed that the quantum yields of photoemission increase markedly with an increase in the photon energy, reaching the order of 10−2 electrons/quantum at hv=10 eV. The threshold energies were lower by 1–2 eV. than the molecular ionization potentials. An energy diagram for the external photoelectric process of organic molecular crystals was introduced on the basis of the intrinsic band model. With the aid of this diagram, the band gap energies, Δε, were obtained from the observed values of photoemission. The values of Δε thus obtained were in fairly good agreement with those based on the results of the temperature dependence of semiconduction.