Effect of Electric Fields on the Emission of Photoelectrons from Oxide Cathodes

Abstract

A study has been made of the photoelectric emission obtained at room temperature from the equipotential oxide cathodes commonly employed in radio receiving tubes. With a double monochromator, long-wave limits were determined by plotting the currents as a function of the wave-length of the incident radiation. The threshold frequencies decreased with increase in the accelerating potential according to the relation, $\ensuremath{\nu}={\ensuremath{\nu}}_{0}\ensuremath{-}b{E}^{\frac{1}{2}},$ in which ${\ensuremath{\nu}}_{0}$ is the "zero-field" threshold and $E$ the electric field at the cathode surface. It is shown that the observed lack of saturation in the photo-currents may be ascribed to a decrease of work function with increase of applied field. The tendency to nonsaturation is greatest for light of longest wave-length. Calculations show that the photoelectric equation of Houston satisfactorily represents the variation in numbers of emitted electrons with color of illumination, and with change of threshold. Some comparisons are made of the photo-emission and the thermionic currents obtained from the same cathode surface. There is a change in photoelectric threshold with activation. Preliminary measurements of thermionic work functions confirm the theory in both cases the emission of electrons takes place from the outermost layers of the oxide coating.