Photo-electric Threshold of Single Bismuth Crystals

A bismuth single crystal grown by Bridgeman's method was mounted in a specially constructed, highly evacuated bell jar. By means of external magnet control it was possible to cut the specimen along a crystal face and advance it into position in a Faraday cylinder. Photo-electric current fatigue curves were taken for the 2537A line under three different vacuum conditions. It was found that the fatigue was smaller, the higher the vacuum. The long wave-length limit was determined by plotting the current per unit intensity as a function of the wave-length of the mercury arc used. The threshold was determined under the two following conditions: first, after the new surface had aged an hour in a vacuum of the order of ${10}^{-6\mathrm{}}$ mm; and second, immediately after cutting a new face in a vacuum of the order of ${10}^{-7\mathrm{}}$ mm. The two values were 2567A and 2835A respectively. Similiar observations on the polycrystalline form yielded the values, 2560A and 2830A. Experiment showed that the threshold changed rapidly from 2835A to 2567A in the low vacuum. It was found that this shift was not due to oxygen, nitrogen, or carbon dioxide, the chief constituents of air, but to some other source of contamination which was not located.