Ionization Efficiency of Ultra-Violet Light in Caesium Vapor

Efficiency of photo-ionization in caesium vapor at 166°C as a function of wave-length.—Light from a quartz mercury arc after being resolved by a quartz spectrograph was passed through a tube containing caesium vapor. The energy of the radiation was measured with a Coblentz thermopile. The photo-ionization of the vapor was determined by drawing out the positive ions to a plate and measuring their number with an electrometer. The vapor pressure of the caesium was obtained by interpolation between existing data. The pressure at 166°C was taken as 0.027 mm which corresponded to an atomic concentration of 5.96×${10}^{14}$ atoms per cc. The efficiency of ionization $B$ (the Einstein probability of absorption coefficient) was found to increase from 0.17×${10}^{-10\mathrm{}}$ ions per atom per erg at 3345A to 3.4×${10}^{-10\mathrm{}}$ at 3135A. The theoretical threshold is at 3184A. For shorter wave-lengths $B$ diminished to a minimum of 1.2×${10}^{-10\mathrm{}}$ at 2800A and then increased slowly for still shorter wave-lengths. The estimated possible error in $B$ is 5 percent in relative values and 25 percent in absolute values.