Single and double ionization of atomic oxygen by electron impact

A pulsed crossed beam technique incorporating time-of-flight spectroscopy (previously developed in this laboratory) has been used to study the single and double ionization of ground-state oxygen atoms produced by an iridium tube furnace source. Relative cross sections sigma 1 and sigma 2 for single and double ionization have been determined in the respective energy ranges 14.1-2000 eV and 90-2000 eV which are wider than any previous measurements. These values have been normalized by reference to the absolute values of sigma 1 previously measured by Brook et al at energies (50-997 eV) where the possible influence of an unknown admixture of long-lived excited atoms in their experiment should be small. However at energies below about 25 eV our values of al become much smaller than the values of Brook at al. Above 40 eV, the present ratios sigma 2 / sigma 1 are in satisfactory agreement with other measurements which extend to 400 eV. Our values of al have been compared with selected theoretical predictions. Some evidence of structure in the energy dependence of sigma 1 in the region of the cross section maximum is consistent with predicted contributions from inner-shell 2s ionization and from autoionization in addition to outer 2p electron removal.