Electron Photoemission from Anthracene Crystals

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Abstract
The electron photoemission (intensity dependence and the electron photoemission spectrum from 2600 to 4600 Å) has been measured for anthracene crystals provided either with a gold paste or with an electron injecting contact. No major differences have been found between the positions of the spectra maxima observed from crystals void of trapped electrons or containing a considerable space charge. Photoemission maxima are observed at wavelengths corresponding to maxima in the absorption coefficient, though in addition a maximum in electron photoemission at ∼3050 Å is observed. A kinetic analysis of the experimental results indicates different rate-determining processes are involved for electron photoemission in the region 2600–3300 Å compared to 3300–4100 Å. In the region 2600–3300 Å two competing processes determine the electron emission characteristics; a charge-transfer exciton–charge-transfer exciton annihilation together with charge-transfer exciton–trapped carrier interactions. The relative importance of each of these two mechanisms depends upon the density of space charge present in the crystal. The lifetime of this charge-transfer state involved is found to be ∼10−9 sec and its energy at least 4.1 eV above the anthracene crystal ground state. Singlet exciton–singlet exciton annihilation is found to be the rate determining process for photoemission between 3300–4100 Å. No evidence for a direct trapped carrier–photon interaction has been observed.