Photoemission of Electrons from Alkali and Alkaline Earth Metals into Anthracene Crystals

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Abstract
We have found that photoemission of electrons from cesium, sodium, calcium, and magnesium into anthracene crystals can be observed. This implies that the interfacial complex layers are thin enough not to modify the energy distribution of photoexcited electrons passing from the metal to the crystal. The main interfacial property is related to surface states which are able to trap electrons if the energy separation of the Fermi level of the metal from the edge of the lowest conduction band of anthracene is lower than the trapping energy. A maximum trap depth of 1.30 ± 0.2 eV is estimated. The occupation of the surface states therefore depends on the work function of the metal contact. These surface charges modify the threshold energies for photoemission from the metal to the crystal. They can be released optically thus giving rise to a component of the photoinjection current displaying a spectral response curve which is identical with the optical absorption of anthracene mononegative ion. The spectral response curve for injection of photoexcited metal electrons to the anthracene crystal is interpreted as a sum of two contributions: (i) injection into the narrow first conduction band and (ii) injection into a broad second band located 0.55 ± 0.05 eV above the first one.