Photo-Ionization and Relative Absorption Probabilities of Caesium Vapor

Abstract

Measurement of photo-electric ionization in gases.—The ${\mathrm{method}}^1$ depends upon the partial neutralization of the negative space charge around a hot wire cathode by the ions produced by radiation. The change in thermionic current is proportional to the product of the intensity of radiation, number of atoms of vapor per unit volume, the Einstein probability coefficient $B_{\nu }$, a quantity $R$ representing the number of electrons released by a single ion and certain numerical factors relating to the geometry of the apparatus. The magnitude of $R$ is of the order ${10}^4$ to ${10}^6$ depending upon the vapor pressure and other characteristics of the tube. Under constant experimental conditions and when the change in current upon illumination is small compared to the total thermionic current, $R$ assumes a definite fixed value. Hence the change in current per unit intensity at different frequencies gives relative values of $B_{\nu }$.