Electron-impact excitation of the second positive band system (C 3 Pi u-->B 3 Pi g) and the C 3 Pi u electronic state of the nitrogen molecule.

Abstract

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