Vibrational-Level Distribution in Electron-Impact Excitation of the N2C3∏uState: Its Disagreement with the Franck-Condon Approximation

Validity of the Franck-Condon approximation is experimentally examined for electron-impact excitation of the C 3 ∏ u state of N 2 molecule. The relative excitation cross-sections for associated vibrational levels up to υ=4 at an electron energy of 48 eV are derived with measuring the emission band intensities of the second positive system. The result is compared with the prediction by the approximation, and appreciable discrepancies are found between them. In particular, the ratio σ υ=3 /σ υ=0 of the cross-sections obtained is by about a factor of two larger than the corresponding ratio of the Franck-Condon factors.