Evaluation of electron impact excitation of N₂ X ¹Σ_g⁺(0) into the N₂⁺ X ²Σ_g⁺(v), A ²Π_u(v), and B ²Σ_u⁺(v) states

Abstract

Published experimental integrated electric dipole photoionization oscillator strengths (fij) have been applied to the determination of absolute electron impact excitation cross sections of the N2 X 1Σg+(0) state into the N2+ X 2Σg+(v), A 2Πu(v), and B 2Σu+(v) states. The required relative electron impact cross section shape functions from threshold to energy values, limited by the imposition of relativistic effects at the high end, are established in this work by extracting accurate analytic functions from a critical review of extensive published experimental results. The cross sections examined here are important benchmarks used in establishing absolute values for critical rate processes in aeronomy. The determination of the absolute cross sections using the derived fij is accomplished through the physical relationship to a modified Born approximation analytic function that defines the electron impact shape functions. The zero order term of the analytic function is established in value relative to the remaining terms through shape analysis of the experimental electron impact results. The value of fij fixes the absolute value of the zero order term, determining the cross sections over the entire electron impact energy range. It is argued that this methodology is the most accurate approach at this time because of the existence of the intrinsically more accurate photometrically dermined fij. The critical quantities in atmospheric process applications are the branching ratios of excitation into the N2+ X 2Σg+, A 2Πu, and B 2Σu+ states. The N2+ (X 2Σg+, A 2Πu, B 2Σu+) partitioning within the total ionization cross section has been a source of disagreement in the literature, in spite of the fact that substantial effort has been expended in establishing acceptably accurate values. The present work establishes accurate cross sections for these states over the entire practical energy range required for research in atmospheric N2 physical chemistry. The accuracy of the results obtained here are verified by consistency with the most reliably established past experimental electron impact cross sections, in agreement with the independently established total N2+ cross section. We provide analytic collision strengths for the vibrational levels of the N2+ X 2Σg+, A 2Πu, and B 2Σu+ states, allowing establishment of rate coefficients for the system, at an estimated error ≤±8%.

First negative system of N<sub>2</sub><sup>+</sup> in aurora: simultaneous space-borne and ground-based measurements and modeling results

“…The best agreement between the experimental data and calculated intensity is obtained by using cross sections from Shemansky and Liu (2005) together with spectroscopic parameters from Gilmore et al (1992). Comparison between the measured intensity of 427.8 nm emissions and the modeled emission using cross sections from Shemansky and Liu (2005), Van Zyl and Pendleton (1995) and Doering and Yang (1997).…”

“…The simultaneous measurements of the 427.8 nm emission by ALIS and Reimei imagers show excellent agreement, indicating that the calibration of the two instruments is correct. Comparison of the 427.8 nm emission intensity calculated using the incident electron flux measured by the Reimei particle instruments with intensities measured by the optical imagers show that the best match is reached with the cross section from Shemansky and Liu (2005). …”

“…It is generally accepted that the partial ionization-excitation cross section of a certain electronic state of the molecular nitrogen ion is obtained from the total single ionization cross section by scaling according to the branching ratios (Shemansky and Liu, 2005). While the total ionization cross section is measured with high accuracy, the branching ratios for X 2 + g , A 2 u and B 2 + u are under debate.…”

“…Comparison between the measured intensity of 427.8 nm emissions and the modeled emission using cross sections from Shemansky and Liu (2005), Van Zyl and Pendleton (1995) and Doering and Yang (1997). Right: Linear regression plot between experimental and modeled intensities.…”

“…Another analysis by Van Zyl and Pendleton (1995) using emission cross section measurements shows that the contribution is 14.5 %. Shemansky and Liu (2005) established the cross sections for electron impact ionization-excitation of N 2 (X + g ) molecules into N + 2 (X 2 + g ), (A 2 u ) and (B 2 + u ) states using ionization oscillator strengths derived from photoionization measurements, claiming that this method is the most accurate approach. They compare their own estimated values with the Published by Copernicus Publications on behalf of the European Geosciences Union.…”