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×{}10¹⁴ atoms per cc. The efficiency of ionization B (the Einstein probability of absorption coefficient) was found to increase from 0.17×{}10^-10 ions per atom per erg at 3345A to 3.4×{}10^-10 at 3135A. The theoretical threshold is at 3184A. For shorter wave-lengths B diminished to a minimum of 1.2×{}10^-10 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.Photoelectric efficiency from a caesium-gold alloy surface.---It was incidentally observed that caesium alloyed readily with a gold electrode and that the resulting surface was very active photo-electrically. The photo-sensitivity varied from 0.5×{}10⁸ electrons per erg at 6100A to 2.0×{}10⁸ at 2800A.