Vibrational populations of the excited states of N₂ under auroral conditions

Abstract

The equations of statistical equilibrium for the population and depopulation of the electronic states of N2 have been solved for steady state normal auroral conditions. All vibrational levels for the seven lowest triplet electronic states (A, B, W, B', C, E, D) and the three lowest excited singlet electronic states (a, a', w) of N2 were included in the calculations. Recently measured auroral secondary electron spectra were employed to calculate the rates for electron impact excitation. The transition probabilities, electron impact excitation cross sections, and vibrationally dependent quenching rates used were a combination of the best available experimental and theoretical data. The results of these calculations are the absolute vibrational populations of all the electronic states. Intrasystem cascading and vibrationally dependent quenching were found to be important in determining the vibrational populations in the lower singlet and triplet excited electronic states. The predicted vibrational populations for the A and B triplet states agree well with the available experimental data. These calculations predict that a fairly large density of N• in excited roetastable states is produced under auroral conditions. Detailed knowledge of the populations processes and emission properties of the valence excited states of Ns is required inorder to obtain a thorough understanding of auroral phenomenology. A significant fraction of the auroral energy incident at the top of the atmosphere appears either as emissions from Ns [Vallance Jones, 1971a, b, 1974] or in the form of metastable Ns electronic states which then participate in energy transfer processes with other atmospheric species. The population processes and emission characteristics for the Cailu state appear to be well understood, since there is now general agreement between auroral measurements of the second positive system and theoretical predictions of the band system intensities. However, no detailed analysis of the population processes for the valence singlet states of Ns under auroral conditions has been given that includes the specific contribu-ments of the population in the lowest levels of the B state under auroral conditions and conclude that their results agree better with the predictions of Cartwright et al. [1971, 1973] than with those of Shemansky et al. [1971]. These recent measurements appear to confirm the earlier predictions that intrasystem cascading is an important mechanism in determining the vibrational populations of certain singlet and triplet electronic states in Ns under auroral conditions. Recent laboratory measurements of the B state vibrational population under discharge conditions [Chen and Anderson, 1975] have also verified the predictions of substantial cascade contributions to the 13 lowest levels of the B state. The purpose of this paper is to present detailed vibrational populations for all the singlet and triplet excited states of Ns, within 12.5 eV of the ground state, excited under auroral tions from the various cascade process...

Role of excited N2 in the production of nitric oxide

“…X 1 S g + tran- [Degen, 1982] (red pluses) and for 110 km [Eastes and Sharp, 1987] sitions terminate in X 1 S g + (n 00 = 7). The main point is that despite the current A 3 S u + populations being less than previously calculated [Cartwright, 1978], the unquenched populations are larger than calculated in another model, so there is not a consistent trend that they are underestimated.…”

“…It is similar to that of Morrill and Benesch, who used the transition probabilities of Gilmore et al However, the densities are about half of those calculated by Cartwright, with only about 10% of this discrepancy being due to the changes in the auroral spectrum and updated cross sections. The densities of Morrill and Benesch are also larger than the current values, but this is not an independent determination because they used excitation rates from Cartwright [1978].…”

“…These intervals are too long for some fast reactions, for which an equilibrium calculation is performed at each time step. Cartwright [1978] and Morrill and Benesch [1996]. The shape of the distribution differs from that of Cartwright due to the use of updated transition probabilities [Gilmore et al, 1992].…”

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“…An important result from Figures 4a and 4b is that the sprite induced VDF of N 2 (X 1 S g + , v) at 78 km is far from equilibrium since there is no Boltzmann (nor Treanor [Caledonia and Center, 1971] can clearly be seen in Figure 5 that the population of each of the considered vibrational levels of N 2 (A 3 S u + ) reaches its maximum value during the pulse phase (time < 5 ms) and, afterward, they remain constant up to about one millisecond to, later on, exhibit a decrease that becomes sharper for the highest vibrational levels. During the pulse (time < 5 ms), the dominant production channels of the lowest vibrational levels is the prevailing production mechanism of the middle and upper vibrational levels (from v″ = 5 to v″ = 16) of N 2 (A 3 S u + ) [Cartwright, 1978].…”

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“…These transi-775 tions can potentially affect the distribution of emit-776 ted radiation in the LBH band (as was found for the 777 Earth's atmosphere by Eastes and Dentamaro, 1996), 778 which are some of the main emission bands observed 779 in Titan's atmosphere. Electronic quenching rates for 780 the a ′ -state are taken from Cartwright (1978). The 781 theoretical calculations also allow us to include the 782 cross manifold collision rates due to spin-orbit inter-783 actions.…”