The Effect of Intense Electric Fields on the Photo-Electric Properties of Metals

The photoelectric effect from thin films of potassium and oxygen on tungsten has been studied as a function of strong accelerating fields. Fields as high as 63,000 volts/cm were used which shifted photoelectric thresholds towards the red, the shifts being approximately proportional to the square root of the applied fields. An applied field of 36,000 volts/cm removed the threshold for a pure potassium layer on tungsten from 5620A to 5880A. A film of potassium on a very thick layer of oxygen on tungsten showed a threshold at 6800A which did not vary with applied accelerating fields. However the magnitude of the emission increased with the field suggesting that thick oxygen layers are rough and applied fields over most of such surfaces are much smaller than calculated from the geometry of the electrodes. A layer of potassium on a thin layer of oxygen on tungsten showed a threshold at 7350A for small applied fields which shifted to 7575A for a field of 18,600 volts/cm. A film of potassium on a thinner layer of oxygen---perhaps less than a monatomic layer---exhibited a threshold in small fields at 5830A which was shifted to 5960A by an applied field of 18,600 volts/cm. From the observations of the variations of the shifts with applied fields calculations of the surface fields were made after the manner of Becker and Mueller. It was found that outside the film of potassium on a thin layer of oxygen on tungsten the field followed closely the Schottky image law in the range 1.5×{}10^-6 cm to 10^-5 cm from the surface. The pure potassium film on tungsten exhibited surface fields which were closely image fields between 8×{}10^-7 cm and 1.5×{}10^-6 cm from the surface but which departed from the image law at greater distances. These observed departures were about equal to the image fields and were much smaller than the surface fields at like distances outside thoriated tungsten filaments as recorded by the thermionic measurements of several observers. The surface fields in excess of the image fields are ascribable to inhomogeneity of the surfaces, regions of different work functions having linear dimensions of the order of magnitude of 10^-5 cm.Shifts of photoelectric thresholds by strong accelerating fields are of particular theoretical interest for they involve changes of the work function of a surface without alterations of the other important characteristics of the metal. It is found that the form of the photoelectric sensitivity versus frequency curve remains unchanged over the range of observations and shifts along with the thresholds in intense fields. Thresholds are not sharp but approach the frequency axis tangentially. These observations are in excellent agreement with the theory of the photoelectric effect based on wave mechanics and the Fermi-Dirac distribution of the electrons in metals worked out by Wentzel and modified by Houston.