The Photo-Electric Response of Potassium at Low Temperatures

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Abstract
Variation of photo-electric currents from potassium with temperature, 20° to -180°C, for the visible spectrum.—Carefully purified potassium in a highly evacuated tube surrounded by a double walled container for liquid air, insulated by wool, was illuminated with monochromatic light of constant intensity. As independently observed by Ives, the current wave-length curve at -180°C is shifted to shorter wave-lengths with respect to the curve at 20°C, the shift decreasing from about 500 A at the long wave limit 6500 A, to 80 A in the blue. These shifts are reversible and reproducible. Monochromatic heating and cooling curves run approximately parallel to the temperature axis except between -80° and -100°, where they show a sudden change. This region is where a change of crystal structure is known to take place. The shifts observed, then, seem to be due in part, at least, to the change of crystal structure. This point needs further study. Slight sudden changes of temperature produced irregular effects which are not understood; they may possibly be associated with the condensation of new vapor or with cracks in the surface.