The Photoelectric Properties of Tantalum

Tantalum was carefully outgassed and it's photoelectric properties studied after stable conditions were reached. Heating 1000 hours at temperatures up to 2200^∘{}K produced an apparent stable condition of the surface. Curves, showing the variation of the photoelectric current as a function of the temperature, are plotted for different wave-lengths. For wave-lengths near the threshold, there is a great increase in the photoemission with increasing temperature. With decreasing wave-lengths, this variation becomes very much smaller. Extrapolated values from the F(λ) curves show the long-wave limit to be at 20^∘{}C, 2750A; and at 700^∘{}C, 2825A. Further heat treatment at temperatures up to 2500^∘{}K produced a final stable condition. Here again are plotted curves showing the variation of the photoelectric current as a function of the temperature for different wave-lengths. In this case, the great increase in the photoelectric sensitivity with temperature for wave-lengths near the threshold becomes smaller as the wave-lengths used decreases, finally reaching negative values for wave-lengths more than 300A shorter than the threshold value. Extrapolated values from F(λ) curves taken at 293^∘{}K and 973^∘{}K show the long-wave limit at the respective temperatures to be 3050A and 3160A. From curves plotted according to Fowler's theory, the true threshold wave-lengths were found to be as follows: (1). Fortantalum in first apparent stable condition (average=2742A (4.50 volts). (2). For tantalum in final stable condition (average) = 2974A (4.15 volts).