ELECTRON PHOTOEMISSION FROM ANTHRACENE CRYSTALS

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.