Direct experimental measurement of electron impact ionization‐excitation branching ratios 4. Branching ratios and cross sections for O2+ at 100 eV

The 100‐eV ionization of molecular oxygen has been studied by the (e,2e) electron‐electron coincidence method. Ionization to the ground state and five excited states of O2+ has been detected. Of these six electronic states, ionization to three, the X2Πg, a4Πu, and b4Σg− states, was found to account for approximately 95% of the O2+ production. Relative triple differential cross sections for the production of O2+ in these three states were measured as a function of ejected electron energy, ejected electron angle, and incident electron scattering angle. Ionization to the three higher energy states, B2Σg−, 2Πu, and c4Σu−, was found to be much weaker, and it was only possible to estimate the branching ratios to these states. Ionization to the A2Πu state, the upper state of the second negative bands of O2+, was not observed, as this state is only weakly excited and is overlaid by the stronger a4Πu state. The branching ratios were found to be 0.540, 0.297, and 0.122 for the X2Πg, a4Πg, and b4Σg− states, respectively. The total branching ratio for the B2Σg−, 2Πu, and c4Σu− states was 0.042 (all values ±20%). The branching ratio to the A2Πu state was estimated to be <1%. The cross sections for the X2Πg, a4Πu, and b4Σg− states were calculated using the previously measured O2+ production cross section for 100‐eV electrons on O2. The results were 92.2 (±20%) × 10−18 cm2, 50.8 (±20%) × 10−18 cm2, and 22.1 (±20%) × 10−18 cm2, respectively. The b4Σg− state cross section was in satisfactory agreement with the recalculated optical fluorescence data on the O2+ first negative band emission cross section of 37.1 (±25%) × 10−18 cm2 [McConkey and Woolsey, 1969].