Effect of heat treatment on the photo-currents from Pt.—The total photoelectric emission from a strip of Pt foil excited by the full radiation from a quartz mercury arc was studied while the strip was put through an extended heat treatment in pressures as low as ${10}^{-8\mathrm{}}$ mm Hg as read on an ionization gauge. Prolonged heating at 1200°-1400°C caused the photo-current to decrease to a final value which could not be further reduced by additional heating for as long as 300 hours at temperatures up to the melting point. The photo-current was found to increase spontaneously from the low values observed immediately after a heating interval to much higher values if the strip was allowed to remain at room temperature for a short time. After thorough outgassing of the strip and the tube however this "recovery" effect finally disappeared.

Effect of heat treatment on the long wave-length limit of Pt.—The long wave-length limit was determined by using filters of dilute acetic acid in a fluorite cell to cut out the shorter Hg lines. The threshold was found to shift during outgassing from above 2500A to a final steady value of 1958±15A. This is at variance with Suhrmann's value of 2675A but is shown to be in agreement with the work of Tucker and Woodruff. Pressure readings taken with an ionization gauge of high sensitivity confirm the conclusions (a) that decreasing photo-currents are caused by the evolution of absorbed gases, (b) that increasing photo-currents (recovery effect) are due to the adsorption of gas by the cool Pt surface, and (c) that the final low values of the photoemission are characteristic of the gas-free Pt. Heating an outgassed strip in air at 0.015 mm pressure caused the photo-currents to disappear; heating in hydrogen at the same pressure caused them to increase. They were brought back to values characteristic of the completely outgassed state in each case by heating for 30 sec. at a pressure of ${10}^{-6\mathrm{}}$ mm.

Influence of temperature on the photo-emission from Pt.—The photoelectric effect of Pt is found to be independent of the temperature only in the region below 500°C. At higher temperatures up to 1200°C the photocurrents increase considerably with temperature and the threshold shifts slightly to the red. Several explanations of the effect were tested and are discussed, the conclusion being that it is a genuine temperature effect characteristic of the metal itself and due to the increase in the thermal energies of the "free" electrons which may become appreciable at the higher temperatures.

• Received 1 June 1926

DOI:https://doi.org/10.1103/PhysRev.29.451