Electron-impact excitation of the singlet states of N2. I. The Birge–Hopfield system (b1πu–X1Σ+g)

Abstract

The absolute emission cross sections for the excitation of the Birge–Hopfield bands (b1πu–X1Σ+g) by electron impact on N2 from threshold to 500 eV are reported. With the exception of the v′=1, 5, and 6 vibrational levels, all other b1πu levels are found to predissociate with a specific predissociation branching ratio, ≳0.99. This forbidden process is a major source of the nitrogen atoms produced by electron-impact dissociation of N2. Quite remarkably, the b1πu (v′=1) state is only slightly predissociated (<10%). The emission cross section for the v′=1 level has a maximum value of 2.3×10−18 cm2 at 35 eV. The absolute transition probabilities, emission cross sections, and the variation of the electric dipole moment with internuclear distance for 21 BH (1, v′) bands are given.