Plasma preconditioning and the role of elevated vibrational temperature in production of excited N2 vibrational distributions

Publisher Website
Google Scholar
Abstract
We have undertaken a series of investigations of excitation and decay processes in molecular nitrogen in pulsed electric discharges under conditions relevant to those of the aurora. Previously, we had observed the continuous evolution of the spectrum of the N2(B³Πg) → N2(A³Σu+) (1 PG) with time and under the influence of collisions after the initial direct electron excitation. In the present investigation, we find that there are progressive changes in the condition of the gaseous medium which occur during the time interval between pulses. At all but the very lowest repetition rates, the sample atmosphere is found to be “preconditioned” at the beginning the excitation pulse by virtue of its previous excitation‐deexcitation history. One possible component of a preconditioned medium is the presence of electronically excited metastables. Another is the retention of excitation in the ground state vibrational manifold. Our observations indicate N2 ground state vibrational temperatures of greater than 3000 K just prior to the beginning of the excitation pulse when conditions for preconditioning are favorable. In addition, our model results indicate that currently accepted N2(B) quenching coefficients are not adequate to explain our observations during the afterglow following the discharge. Ground‐based and rocket measurements of the auroral and airglow spectra have revealed alterations in the distribution of nitrogen molecular band emission intensities only very recently. We are led to believe that most of these phenomena being uncovered in the laboratory are likewise occurring in the aurora, to varying extents depending on conditions and will become evident as observational technology advances. We include a discussion of some conditions under which deviations in the distribution of excited N2 band intensities have been observed in the aurora.