The Ionization of Oxygen by Electron Impact as Interpreted by Positive Ray Analysis

Using an apparatus previously described, in which ions formed by impact of electrons of definite energy are analyzed by Dempster's positive ray method, the relative numbers of $0_1^{+}$ and $0_2^{+}$ are measured at different pressures and electron energies. Over a range of pressure from ${10}^{-5\mathrm{}}$ to ${10}^{-2\mathrm{}}$ mm of mercury in pure oxygen, and over a large range of partial pressures of oxygen in mixtures with helium and with argon, the ratio $\frac{{0_2}^{+}}{{0_1}^{+}}$ remains practically constant. Evidence is thus given that $0_2^{+}$ and $0_1^{+}$ are formed by independent primary processes and, therefore, that an impact electron of sufficient energy can either ionize the molecule and form $0_2^{+}$ or dissociate the molecule forming $0_1^{+}$. The ionization potentials were determined: $0_2={0_2}^{+}+\epsilon$; 13 volts and $0_2={0_1}^{+}+0+\epsilon$; 20 volts. The latter, together with the ionizing potential of atomic oxygen, gives 6.5 volts (150,000 cal. per mol) for the heat of dissociation of oxygen. Both molecular and atomic negative ions were found. The above results are correlated with band-spectra data, with the recent theory of Franck on dissociation by absorption of radiation, and with the structure of the oxygen molecule.