Single and Double Ionization of Argon by Electron Impacts

The magnetic deflection method, described in the preceding paper by H. D. Smyth, has been applied to a study of the ionization of argon, and the minimum potentials for the production of singly and doubly charged ions have been separately observed. Assuming that for ${\mathrm{A}}^{+}$ to be 15.2 volts, the correction for the apparatus was determined and the corrected minimum potential for the production of ${\mathrm{A}}^{++}$ ions was found to be 45.3±1.5 volts. No ${\mathrm{A}}^{3+}$ nor $\mathrm{A}_2^{}{}_{}{}^{+}$ ions were observed. Spectroscopic evidence has shown that the blue spectrum is excited by impacts of 34 volts, and it has been assumed that doubly charged ions are produced at this potential. Four ways of reconciling the direct evidence of this paper with the spectroscopic evidence are suggested and discussed. (1) A cumulative process may produce ${\mathrm{A}}^{++}$ ions with only 34 volts in discharge tubes with high current density, though not in the author's tube in which conditions were unfavorable. (2) The 34 volt lines may represent only a partial excitation of the spark spectrum, double ionization not being required. (3) This potential (34 volts) may correspond to the limit of spectral terms of negative values, involving the simultaneous excitation of two electrons. (4) A more closely bound electron with an ionizing potential of 34 volts may undergo transitions without disturbing the electron whose transitions produce the arc spectrum.