Direct experimental measurement of electron impact ionization‐excitation branching ratios: 3. Branching ratios and cross sections for the N2+X2Σg+, A2Πu, and B2Σu+ states at 100 eV

Abstract

The three electronic states of N2+ which are important to aeronomy are the X 2Σg+, A 2Πu, and B 2Σu+ states. The transitions X 2Σg+ ← A 2Πu, and X 2Σg+ ← B 2Σu+ are responsible for the prominent Meinel bands and first negative bands which occur in the aurora and airglow. In this paper, we use new electron‐electron coincidence (e,2e) measurements to extend our previous work on the B 2Σu+ state cross section to give a complete set of branching ratios and cross sections for N2+ production in the three electronic states given above by 100‐eV electron impact on N2. An improved electron‐electron coincidence apparatus has allowed the direct measurement of the cross section and branching ratio for the X 2Σg+ state for the first time. The cross sections for the X and A states agree within experimental error with the existing optical excitation results in the literature, but the error bars on the present measurements are smaller than those for the optical experiments. The branching ratios for the N2+ X and A states at 100 eV are 0.448±0.033 and 0.453±0.033, respectively. The corresponding 100‐eV excitation cross sections for the X and A states deduced from the total N2+ 100‐eV electron impact ionization cross section which is known from other measurements are 86.9±7.0 × 10−18 cm2 and 87.9±7.0 × 10−18 cm2, respectively. The B state results, which have been discussed in detail in a previous paper, are branching ratio of 0.099±0.017 and excitation cross section of 19.2±3.3 × 10−18 cm2. These values are significantly lower than the result deduced from the widely accepted optical emission cross section of Borst and Zipf [1970] but in satisfactory agreement with the “best” value for the cross section of 23.6 × 10−18 cm2 previously deduced from a combination of optical and (e,2e) results.