Interferometric measurements of linewidths and spin doubling in the N_2^+ first negative band system in free-jet expansions

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Abstract
The lower rotational R and P branch lines of the N2+ first negative band, 0–0 vibrational transition, have been studied interferometrically. Spectral source temperatures less than 20°K were obtained using a free-jet expansion. The first negative system was produced with a beam of 30 keV electrons and the spin doubling and spectral line shape of a given rotational line was observed with a Fabry-Perot interferometer. It was found that the values of the spin-doubling constants, as determined independently from R and P branch measurements, were not consistent, i.e., different values were obtained from the R and P branch data. This implies that the interpretation of the doublet splittings as spin doubling is incomplete. Furthermore, it was found that the measured linewidths indicated temperatures of about 70°K greater than the actual gas temperature. These discrepancies are attributed to hyperfine structure.