The Ionization of Potassium Vapor by Light

Abstract

Photo-ionization of potassium vapor by ultra-violet light to $\lambda$ 1850 Å.—(1) Jet method. To insure that the light was not appreciably absorbed before it reached the vapor under observation, and to reduce thermionic and photoelectric emission to a minimum, a tube was used in which a jet of potassium vapor was directed from a heating chamber into the vacuum observation chamber (pressure about ${10}^{-4\mathrm{}}$ mm.) and was there condensed on the walls, while a carefully diaphragmed beam from a quartz mercury arc was directed through the jet into a conical light trap. The presence of positive ions in the illuminated vapor was determined bv means of the usual accelerating and retarding electric fields. (2) Variation with wave-length. By the use of a series of screens, the long wave-length limit for ionization was found to be between 2,800 and 3,100 Å., in agreement with the limit, 2,856 Å., calculated from the ionization potential by means of the quantum relation. No positive ions were obtained when the radiation was limited to wave-lengths greater than 3,100 Å. or when the jet was intercepted by a sliding shutter. As the wave-length was decreased below 2,800 Å., the ionization rapidly increased in amount; being apparently a continuous function of the wave-length. (3) Variation with pressure of the vapor in the jet was found to be linear. (4) Proportion of exposed atoms ionized was computed to be roughly 1 in ${10}^{10}$ for an exposure time of ${10}^{-5\mathrm{}}$ seconds or 1 in ${10}^5$ per second for continuous exposure.