It has been shown recently that efficient electroluminescence may be obtained from thin films formed by the. coevaporation of ZnS and rare-earth fluorides. In order to assess the importance of the fluorine ions in such systems, films have been produced by coevaporating ZnS and rare-earth metals (Tb, Sm, Nd). It is shown that large concentrations of well-dispersed rare-earth ions may also be obtained by this method. For ZnS: (Tb metal) the concentration which optimizes the electroluminescence power efficiency has about the same value as that for ZnS: TbF 3 (2 at. % Tb), and the peak electroluminescence power efficiency is slightly lower than that obtained for ZnS: TbF 3 • In general, the metal-doped systems yield electroluminescence spectra which differ from their fluoride-doped counterparts at a similar concentration. This is taken as evidence that the fluorine ions remain in the vicinity of the rare-earth ion. In the case of ZnS: EuF 3 , it is further suggested that the fluorine ions may help to control the valence of the rare-earth ion to yield Eu 3 + since independent evidence indicates that in ZnS crystals only Eu 2 + is stable. Absolute photoluminescent power efficiency measurements on films of ZnS : (Tb metal) are used to show that a mechanism for electroluminescence based on electron-hole pair formation with subsequent transfer of this energy to the Tb3+ ion may be effectively ruled out; indirect evidence for direct impact excitation of the active ion is thus obtained. For purposes of application, it appears that systems in which the rare-earth ions are introduced by evaporation of the fluorides are superior, since the evaporation of these is simpler and the electroluminescence spectra of such devices are more reproducible.

The temperature dependence (10-160 K) of the phosphorescence lifetimes and intensities of 2,4,5-trimethylbenzaldehyde (TMB) in crystalline durene have been investigated. Two phosphorescence bands (407"5 and 411'2 nm) are observed to grow in with increasing temperature, reach a maximum intensity, and decrease in intensity at higher temperatures. Exponential phosphorescence decays are observed in the 10 to 95 K and 110 to 160 K ranges, while non-exponential decays are observed in the 95 to 110 K region. Ultra-violet photolysis, particularly at higher temperatures, is seen to decrease the intensity of the infra-red carbonyl stretching band at 1703 cm -x, but not the one at 1684 cm -1. A model in which two inequivalent sets of TMB molecules in the durene lattice undergo their own unique photochemical and photophysical processes is suggested to explain the results. It is proposed that within each site the lower, predominantly Z~rrr* state of the anti TMB conformer is in thermal equilibrium with the higher-lying 3nrr* state of the syn conformer which is formed by out-of-plane rotation of the aldehyde group in the excited state. In one site, intramolecular photoenol formation occurs via thermal activation from the 3nr* of the syn conformer, while in the other site intermolecular hydrogen abstraction occurs, leading to the simultaneous production of a photoenol and a duryl radical. This model is shown to be consistent with previous knowledge of the primary photochemical processes of TMB in durene.