High energy-resolution studies of electron-impact optical excitation functions. I. The second positive system of N2

The relative emission cross section for the N2 second positive system (0, 0) and (1, 0) bands excited by electron impact has been measured for incident electron energies from threshold to 17 eV. By using a high energy-resolution electron spectrometer coupled to an optical detection system, it was possible to perform the measurements with an incident electron beam having an energy half width of 50 meV (FWHM). Observation of various scattering resonances in nitrogen and helium provided an accurate calibration of the incident electron energy. The maximum of the N2 second positive system (0, 0) band emission cross section (λ3371 Å) was found to be at 14.02 ± 0.05 eV. The maximum for the (1, 0) second positive band (λ3159 Å) was found to be at 14.3 ± 0.1 eV. Two small structural features corresponding to the onset of additional channels for population of the N2 C 3IIu state were observed at 11.48 eV and between 11.8 and 12.3 eV. The first feature was pressure independent and appeared only in the (0, 0) band emission cross section. This feature apparently arises as a consequence of decay of the compound state responsible for the 11.48 eV N2 elastic scattering resonance to the N2 C 3Πu(v′=0) state. The second feature was broad and pressure dependent. It appeared more strongly in the (1, 0) band cross section than in that for the (0, 0) band. Collisional quenching of the N2 E 3Σg+ state via a collision-induced crossing into the vibrational states of the N2 C 3IIu state appears to be the best explanation of this effect.