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 ${\lambda }_0$ 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 ${\lambda }_0$ 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 ${\lambda }_0$ to a longer wave-length, 2770 A, when the tube was baked at 200°C without glowing the foil.