The variation of the photoelectric sensitivity of a molybdenum surface during prolonged heat treatment has been studied over a period of 300 hours of intense heating. Thin ribbons of molybdenum were heated to a temperature of 1325°C in a high vacuum and the photocurrents produced by the light from a quartz mercury arc were measured with a quadrant electrometer. The photoelectric sensitivity was found to increase as the heating progressed, finally reaching a limiting value. The increase in sensitivity was accompanied by a shift in the wave-length limit from approximately 2600 A. U. to approximately 3800 A. U.

The photoelectric work function of molybdenum was determined from the characteristic wave-length limit of the heat treated surface after a condition of stability had been attained. The value of the work function so determined was 3.22±.16 volts.

The thermionic work function of molybdenum was determined for the same speciments. The thermionic emission as a function of temperature was plotted logarithmically according to Richardson's equation. The value of the thermionic work function determined from the slopes of these curves was 3.48±.07 volts. The photoelectric and thermionic work functions were thus found to agree within 0.26 volts.

The variation of photoelectric sensitivity of molybdenum with temperature was observed. The sensitivity was found to increase with temperature, the variation amounting to approximately 30 percent between room temperature and 1000°C.

• Received 11 March 1929

DOI:https://doi.org/10.1103/PhysRev.33.991