The Variation in the Photo-Electric Emission From Platinum

Photo-electric emission from Pt.—The specimens were ribbons 3 cm by 5 mm by.013 mm thick, mounted in a Pyrex tube with a quartz window. Effect of heat treatment on sensitivity. Fresh specimens, cleaned with nitric acid, were insensitive to mercury arc radiation, but became sensitive when heated to moderate temperatures (250°C) by means of an electric current through them, though heating to a much higher temperature in an electric oven was without effect. Prolonged heating (17 hr) at 1300°K with pressure of ${10}^{-6\mathrm{}}$ mm, reduces the sensitivity to light transmitted by quartz to zero. One specimen so treated remained inactive for 2 months. With pressures below ${10}^{-7\mathrm{}}$ mm the sensitivity begins to return as soon as heating is stopped, the rate of recovery depending on the wave-length, the pressure and the previous treatment. A specimen insensitive to mercury arc radiation, was sensitive to the radiation from a tungsten filament within the tube, showing that the insensitivity is due to a shift of the threshold to short wave-lengths. Thermionic measurements gave an increased value for the work function $\varphi$ when the threshold ${\lambda }_0$ was shifted to wave-lengths below that transmitted by quartz, in general agreement with the theoretical relation between $\varphi$ and ${\lambda }_0$. At low pressures, a change of sensitivity in the ratio 1 to 7 was observed without a shift of the threshold wave-length, Effect of electric field. After prolonged heating at ${10}^{-6\mathrm{}}$ mm, small electric potentials, 0.01 to 0.06 volt, are sufficient to prevent the escape of all electrons excited by mercury arc radiation. If the pressure is much lower the electric field shifts the long wave-length limit toward shorter wave-lengths. In both cases photo-activity is not restored when the retarding field is removed but only when an accelerating field is applied. The effect appears to be instantaneous. These results seem to require an adsorbed layer of gas for their explanation. However, the layer formed by heating at high temperatures seems different from that initially present in fresh specimens. This seems to be a retarding ionic layer formed from the residual gas, since at the lowest pressures this seemed to cover only patches of the surface.