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 ${B}_{\ensuremath{\nu}}$, 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 ${B}_{\ensuremath{\nu}}$.Absorption probabilities.---The absorption probability coefficient of a caesium atom for wave-lengths shorter than the $1s$ limit $\ensuremath{\lambda}3184$ decreases much more rapidly with decreasing wave-length than follows from the ${\ensuremath{\lambda}}^{3}$ law of Milne or the ${\ensuremath{\lambda}}^{4}$ law of Kramers although the latter is satisfactory for x-ray absorption. The theory of Becker is in good agreement with the data for the range $\ensuremath{\lambda}3050$ to 2700. An empirical relation of the form ${B}_{\ensuremath{\nu}}={B}_{\ensuremath{\nu}1}{e}^{\ensuremath{-}a({\ensuremath{\lambda}}_{1}\ensuremath{-}\ensuremath{\lambda})}$ where ${\ensuremath{\lambda}}_{1}=3184$, appears to represent the experimental values throughout the observed range $\ensuremath{\lambda}3184$ to 2600. Vapor pressures investigated extended from 5 to 500 bars.Photo-excitation.---Photosensitivity curves between $\ensuremath{\lambda}3900$ and the limit show that appreciable ionization is produced only at the wave-lengths corresponding to the principal series lines, the resolved effect of the individual lines being measurable from $1s\ensuremath{-}4p$ to $1s\ensuremath{-}9p$ with indications of further peaks, the definition of which was limited by the resolving power of the monochromator. The ionization is due to the production of excited atoms which are ionized by atomic collision. The relative magnitude of the photo-ionization at the different lines is in accord with the theory based upon the above hypothesis.