Collision‐induced transitions between the a1Πg, a′ 1Σu−, and w1Δu states of N2: Can they affect auroral N2 Lyman‐Birge‐Hopfield band emissions?

Abstract

In the Earth's atmosphere bright ultraviolet (UV) emissions from the N2 a 1Πg state (Lyman‐Birge‐Hopfield bands) are seen, but the w 1Δu and a′ 1Σu− states, which have similar excitation cross sections, emit weakly if at all at UV wavelengths. Models of the singlet system (a, a′, and w states) have used radiative cascade, which produces transitions between excited states, and quenching, which removes molecules from them, to explain the lack of emission from the a′ and w states. Another process, which has never been properly included in such calculations, is collision‐induced electronic transitions (CIET) between the singlet states. Recent laboratory measurements of CIET involving N2 indicate that the rate constants are comparable to those for radiation, and recent measurements have yielded enough information to allow a preliminary calculation of CIET between the N2 a 1Πg, a′ 1Σu− and w 1Δu states. These calculations show that CIET can be more important than radiation for transitions between the a, a′, and w states in typical aurorae or up to the peak of typical dayglow emission profiles (∼130 km). The inclusion of CIET may allow self‐consistent modeling of observations of the Lyman‐Birge‐Hopfield (LBH) bands, one of the most prominent ultraviolet molecular emissions from the Earth's atmosphere.