Field Dependence of Photoelectric Emission from Tantalum

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Abstract
An experimental study is made of the photoelectric emission from tantalum as it depends directly on accelerating electric field. Observations cover a range of fields from 0 to 90 kv/cm and illumination wavelengths from 248 to 300 mμ. The results are in agreement with the Fowler photoelectric theory modified by a Schottky lowering of the surface barrier. No evidence is found for a periodic deviation from this regular behavior such as exists in its thermionic counterpart, but experimental conditions may have been inadequate for the observation of such a deviation. A new method of threshold measurement at high fields is described and applied to drawn tantalum filaments of circular cross section. The resulting value is 4.16 ev for freshly flashed specimens and it is suggested that this is slightly greater than the lowest work function present in the surface. A Fowler determination in the same field range yields 4.13 ev. The influence of nonuniformity of the surface work function on the field dependence of photoemission and on threshold measurements is discussed.