Ionization of gases by positive ions
With a tube of simple design, positive ions of potassium are driven by controllable accelerating potentials up to 750 volts into neon and argon at various pressures. Small collecting potentials are adjusted in such a way as to eliminate practically all secondary effects due to the ions, enabling the collection of any electrons liberated by ionization in the gases. Pressure ranges between 0.05 and 1 mm give definite variation of ionization with pressure. Curves are given showing the variation of ionization with pressure and the number of new ions formed per positive ion per cm path reduced to 1 mm pressure. Ionization by this process does not offer a sharp initial point, but a measurable amount is present in neon at 100 volts, and in argon between 100 and 150 volts. The efficiency of the effect is decidedly less than for electron bombardment in these gases; it is several fold greater in argon than in neon. In neither gas has the efficiency reached a maximum at 750 volts, although there is evidence in the case of argon that the maximum efficiency may be found at a slightly higher accelerating potential. The pressure range within which the ionization effect shows itself is higher than should be expected from kinetic theory, which seems to indicate the presence of long mean free paths for the positive ions in accord with results of other observers using magnetic analysis apparatus.
©1929 The American Physical Society. Received 15 December 1928. The writer wishes to express his appreciation of the helpful interest given by Dr. R. A. Millikan during this work, and of the critical suggestions offered by Mr. Willy Uyterhoven.