The photoemission from evaporated films of 12 alkali halides has been measured in the 10-to 21-eV energy region. In addition, some preliminary optical-density values were obtained by measuring the transmission of evaporated layers of these compounds deposited on 450A-thick, self-supporting, AI2O3 films. The photoemission yield curves show broad structures correlated with the absorption coefficients. In general, the photoemission is a maximum for photons of energies of about 5 eV greater than the forbidden-gap energy E e , with additional peaks beyond 2 E g . The correlation of the yield with the absorption coefficient is most pronounced in the alkali fluorides, which have relatively low yields, but is less apparent in the other halides, which have peak values of as much as 0.8 electron/photon. A random-walk model of electron scattering is used to estimate the magnitude of the yields and the dependence on the absorption coefficient, with good qualitative agreement with the experimental results, but other processes cannot be ruled out.
The lifetimes of two excited states of iodine centers in KC1 single crystals were measured between 15 and 300°K, following flash excitation. Previous and new results of the fluorescent yield of these same emission processes are presented for comparison. At low temperatures the radiative lifetime is 1.7X10 -4 sec and the yield 100%±50% for the blue-green emission process; for the ultraviolet emission the lifetime is 1.3X10 -7 sec and the maximum yield 70%±35%. In an intermediate temperature range, the radiative lifetime starts to decrease whereas the yield stays constant. At still higher temperatures, the lifetime and the fluorescent yield both decrease rather sharply.
The problem of computing the field or other perturbation at a lattice site due to a random collection of impurity centers is considered. The Fourier transform of the field probability density is shown to be of the form exp[−n̄Ψ(t)], where n̄ is the average concentration of impurities and Ψ(t) is an integral that is a function of the individual center–impurity interactions. The Fourier transform is used to calculate numerous quantities that are functions of the random field. Emphasis is placed on computing luminescent decay forms and the energy-transfer probability from one center to surrounding acceptors, but the application of these calculations to the evaluation of other quantities (e.g., partition functions) is also considered. Graphs of some of the computed functions are presented.
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