EVOLUTION OF $N_{2}$ EXCITED STATE VIBRATIONAL LEVEL POPULATIONS IN A PULSED ELECTRIC DISCHARGE

This research was supported by the Atmospheric Sciences Section. National Science Foundation and the Optical Physics, Division, Air Force Geophysics Laboratory. Address: Institute for Molecular Physics, University of Maryland, College Park, MD 20742Author Institution:Experiments have been conducted to elucidate the processes governing the distribution of population among the molecular nitrogen triplet state vibrational levels under circumstances where molecular collisions are important. The conditions of the experiments are maintained so as to permit the application of the results to an understanding of similar processes in low altitude aurora. Nitrogen gas, at pressures of 0.03 to 3.0 Torr, is excited by means of a pulsed electric discharge which, typically, is supplied with pulses of 12,000 volts and 1,000 amperes and durations of $4 \mu sec$ in a large volume glass container. The radiation issuing from the discharge tube is analyzed by means of time-resolved spectroscopy with particular attention being given to the nitrogen First Positive System, $B^{3}\Pi_{g} \rightarrow A^{3}\Sigma_{u}^{+}$. Dramatic changes have been noted in the relative band intensities of these spectra subject to the influence of the variation of the experimental conditions. In particular, the distribution of population among vibrational levels of the $B^{3}\Pi_{g}$ state evolves with time both during and after the exciting electric pulse with the trend strongly correlated with the number of collisions, independent of pressure. Further, emission from these excited vibrational levels continues at much higher rates than those commensurate with their radiative lifetimes. Both of these observations constitute evidence of the intersystem collisional transfer of excitation. Additional studies show that the initial $B^{3}\Pi_{g}$ (upper state) vibrational population distributions produced during the excitation pulse vary with discharge pulse frequency. An increase of pulse frequency from a low to a high rate gives rise to a trend in the spectrum from one similar to an auroral distribution to one characteristic of a DC discharge. Analysis of these intial populations gives strong indication that excitation is occurring from an elevated vibrational distribution within the ground state. Results imply that this ground state distribution is quasi-Boltzmann with vibrational temperatures of up to $2500^{\circ}K$