Normal Energy Distribution of Photoelectrons from Thin Potassium Films as a Function of Temperature

With a retarding potential between plane parallel electrodes the normal energy distribution of photoelectrons from thin films of potassium on silver and platinum was studied as a function of temperature. The films were both greater and less than monatomic in thickness and were produced by the molecular ray method. Contact potential was determined by the Kelvin method to within less than 0.02 volt. The resulting current-voltage curves are found under optimum geometrical conditions to be parabolic in shape up to the point of zero field within experimental error. This is shown by plotting (current)^ vs. voltage and testing with a straight line, and also by agreement with the theoretical Fowler curve. These results are compared with various theories. Mitchell's prediction for normal energy distribution has a large deficiency in slow electrons. Hill's calculations in which he uses an image barrier instead of the square barrier of Mitchell show that there should be very little deviation of the energy distribution from DuBridge's simple theory. The agreement of the results with DuBridge's distribution suggests that the image barrier is a much more reasonable approximation although the method is insensitive to small changes in the form of the barrier. The effects of temperature are: (1) temperature dependence of the rates of diffusion of potassium through and sublimation from the base metal, (2) irreversible changes with temperature in the state of the potassium surfaces as shown by changes in contact potential and saturation current, (3) reversible changes with temperature in the shape of the Fowler plots, (4) a change of photoelectric current with temperature only 10 to 30 percent of that predicted by DuBridge's theory. (4) is obtained from (3) by determining empirically the optimum temperature at which data should be plotted to give best fits and shifts for the Fowler curve. They range from 90^∘{}K to 175^∘{}K instead of the actual temperatures of 83^∘{}K to 296^∘{}K. The temperature effect seems less with thinner films. No theory for thin films predicts this last effect.