Effect of Heat Treatment on the Photo-Electric Emission from Platinum

Effect of heat treatment on the photo-electric emission from platinum. A thin Pt foil was mounted between heavy leads inside a Faraday cylinder in a pyrex tube with a quartz window sealed to it by means of a graded seal. The threshold or long wave-length limit λ₀ was determined by plotting the current per unit light intensity as a function of wave-length of the Hg arc line used. This threshold was found to vary with the previous heat treatment. Glowing the foil at 900^∘{}C for 8 hours gave repeatedly a value close to 2700 A; glowing for 4 or 5 hours at bright yellow heat decreased the emission greatly and shifted λ₀ to 2475 A. Immediately after glowing for 2 hours at a temperature of over 1400^∘{}C, the emission from the suddenly cooled foil was found to be zero even under the influence of the full radiation of the Hg arc; after 5 minutes the photo-emission began to appear and increased rapidly at first and then more slowly. Evidently the threshold for pure gas-free Pt must be below 1849 A and the effects ordinarily observed must be due to a gas film or some other film formed on the surface. This is also suggested by the shift of λ₀ to a longer wave-length, 2770 A, when the tube was baked at 200^∘{}C without glowing the foil.Variation of photo-electric threshold for platinum with temperature to 600^∘{}C.---After a given moderate heat treatment of the foil, the threshold was found to be the same at 600^∘{} as at 20^∘{}. This indicates that the film responsible for the emission was unchanged and that the threshold under these conditions does not depend on the kinetic energy of the free electrons in the foil.