Electron-impact excitation of the second positive band system (C${\mathrm{}}^3$${\mathrm{\Pi }}_{\mathit{u}}$→B${\mathrm{}}^3$${\mathrm{\Pi }}_{\mathit{g}}$) and the C${\mathrm{}}^3$${\mathrm{\Pi }}_{\mathit{u}}$ electronic state of the nitrogen molecule

Absolute optical emission cross sections have been measured for the second-positive band system of ${\mathrm{N}}_2$, C${\mathrm{}}^3$${\mathrm{\Pi }}_{\mathit{u}}$(v′)→B${\mathrm{}}^3$${\mathrm{\Pi }}_{\mathit{g}}$(v″), for v′=0,1,2,3,4 and v″ as large as 9 produced by electron impact with the nitrogen molecule for incident-electron energies from threshold up to 600 eV. The relative cross sections for each v′ family are in good agreement with the theoretical values. From the measured optical emission cross section, the apparent excitation cross sections for the v′=0,1,2,3,4 vibrational levels of the C${\mathrm{}}^3$${\mathrm{\Pi }}_{\mathit{u}}$ electronic states are determined. A comparison of these apparent cross sections with the relative direct excitation cross sections predicted by the Franck-Condon principle suggests that the population of the C${\mathrm{}}^3$${\mathrm{\Pi }}_{\mathit{u}}$ state in an electron-beam experiment for the v′=0,1, and 2 levels is primarily due to direct excitation with minor contributions from cascade. For the v′=3 and 4 levels, the direct excitation cross sections are much smaller so that a larger percentage population is attributed to cascade. The relative intensities of the various (v′,v″) bands in the electron-beam experiments are also compared with those observed in a dc discharge. © 1996 The American Physical Society.