An investigation of the second negative system of O₂⁺ by electron impact

We have measured the calibrated optically thin spectrum of O 2 from 115 to 300 nm induced by electron impact at 200 eV. Spectra were obtained in a crossed beam configuration at 1.25‐nm resolution. We have observed the second negative system of O 2 + ( A ²Π u → X ²Π g ) in the range 186–300 nm. By comparison with O I (130.4 nm) and using the results of model calculations we estimate the emission cross section for the O 2 + A state at 200 eV to be 2.57±0.57×10 −18 cm². The excitation function from threshold to 400 eV has also been measured at 220 nm and compared to the total ionization cross section. We have used the results of our laboratory measurements to model the contribution made by the second negative system of O 2 + to auroral spectra in the same wavelength range. Observations of the spectral region from 140 to 185 nm show no detectable O or O + transitions with a cross section upper limit of 10 −21 cm².