  We study the electron-impact induced ionization of O$_{2}$ from threshold to
120 eV using the electron spectroscopy method. Our approach is simple in
concept and embodies the ion source with a collision chamber and a mass
spectrometer with a quadruple filter as a selector for the product ions. The
combination of these two devices makes it possible to unequivocally collect all
energetic fragment ions formed in ionization and dissociative processes and to
detect them with known efficiency. The ion source allows to vary and tune the
electron-impact ionization energy and the target-gas pressure. We demonstrate
that for obtaining reliable results of cross sections for inelastic processes
and determining mechanisms for the formation of O$^{+}$($^{4}S,^2{D},^2{P}$)
ions, it is crucial to control the electron-impact energy for production of ion
and the pressure in the ion source. A comparison of our results with other
experimental and theoretical data shows good agreement and proves the validity
of our approach.
