Direct experimental measurement of electron impact ionization‐excitation branching ratios: 1. Results for N2 at 100 eV

A new laboratory experiment has been developed to measure electron impact ionization‐excitation branching ratios for formation of positive ions of atmospheric species in the ground and excited electronic states of the ion. In the experiments described here the distribution of N2+ ions produced in the X ²Σg+, A ²Πu, and B ²Σu+ electronic states by 100‐eV electron impact ionization of N2 has been measured directly for the first time. The experiment is a modification of general purpose (e,2e) electron coincidence ionization experiments which have been developed in recent years to study ionization processes. We have added a second electron energy analyzer to an existing electron energy loss spectrometer so that both the scattered primary and ejected secondary electrons can be detected in coincidence. The final ion state is determined from the energy of the ejected electron and the energy lost by the incident electron. Spectra of coincidence rate versus primary electron energy loss have been obtained at fixed ejected secondary electron energies of 2.5, 5.0, 10.0, and 15.0 eV. Peaks in these spectra are observed corresponding to the three final states of the N2+ ion. The relative peak areas are proportional to the partial branching ratios at each secondary electron energy. Branching ratios at 100 eV integrated over angle and energy variables have been computed from the data. They are 0.50, 0.40, and 0.10 for the X ²Σg+, A ²Πu, and B ²Σu+ electronic states respectively. These values are in excellent agreement with branching ratios obtained by other techniques.