Theoretical study of the A′ 5Σ+g and C″ 5Πu states of N2: Implications for the N2 afterglow

Abstract

Theoretical spectroscopic constants are reported for the A’ ⁵Σ⁺_g and C‘ ⁵Π_u states of N₂ based on CASSCF/MRCI calculations employing large ANO Gaussian basis sets. Our calculated A’ ⁵Σ⁺_g potential differs qualitatively from previous calculations in that the inner well is significantly deeper (D_e ≊3450 cm⁻¹). The potential also has a substantial barrier (≊500 cm⁻¹) to dissociation with a maximum near 4.3 a₀, and then a shallow (≊47 cm⁻¹) van der Waals minimum near 6.3 a₀. We suggest that it is the v=4 and v=5 levels that are involved in predissociating the a ¹Π_g and B ³Π_g states, as opposed to v=0 and v=1 proposed by Verma. The deeper well in the A’ ⁵Σ⁺_g state provides considerable support for the theory of Berkowitz, Chupka, and Kistiakowsky who proposed that A’ ⁵Σ⁺_g is the primary precursor state involved in the yellow Lewis–Rayleigh afterglow of nitrogen. The theoretical D_e for the C‘ ⁵Π_u state is also much larger than previous estimates. The previously unassigned Hermann infrared system (HIR) band positions correspond exceptionally well to our theoretical values for the C‘ ⁵Π_u →A’ ⁵Σ⁺_g transition. This and other considerations lead to a rather convincing assignment of this transition as the HIR system. Einstein coefficients and radiative lifetimes are presented for this transition. Since the natural radiative lifetime of this state is much longer than experimental estimates, the C‘ ⁵Π_u state may derive its actual lifetime through interaction with the C ³Π_u state.