Photo-Ionization of a Gas by a Discharge in the Same Gas

Method. Measurements were made in a tube containing two thermionic units electrically shielded from each other. The first served to give a discharge, the second detected ions produced by the radiation from the first by means of their effect on the space charge.Results. Relatively little photo-ionization is produced by arc radiation, while spark excitation and soft x-radiation give a strong effect. Curves of photo-effect versus discharge voltage show critical potentials which have been interpreted as follows: Ionization of n₂ rare gas shell---Cs, 13.0; K, 19.0: Spark excitation---Cs, 18.5; K, 21.6; A, 32.2; Ne, 48.0: Double ionization---Cs, 21.5; K, 31.8; A, 34.8; Ne, 54.9: Ionization of n₁ rare gas shell---Cs, 39.0; K, 48.0; A, 39.6. The neon II spectrum is excited at 55 volts.It was shown that photo-electric emission from the electrodes was at least ten times as great as the photo-electric emission from the gas, while the current change produced by the space charge effect was more than 2400 times the ion current in argon and caesium, the upper limit being quite indefinite.