Continuous-wave cavity ringdown spectroscopy of the [formula omitted] Meinel system (2, 1) band

Abstract

The A 2 Π u – X 2 Σ g + system of N 2 + was first observed in auroral emissions by Meinel in 1950. Although the N 2 + band system has been reinvestigated since this first spectral study, no laboratory spectrum of the (2, 1) vibronic band has been obtained. We have recently built a continuous-wave cavity ringdown spectrometer, and as a first test of this spectrometer we observed the (2, 1) band of N 2 + in a positive column discharge cell. Many lines of the first positive band system of N 2 ∗ were also identified during the process of assigning this spectrum. The relative intensities of the N 2 ∗ and N 2 + bands were found to change with discharge cell pressure, and so each spectral region was observed at two pressures to aid in distinguishing the spectra of the two species. We have assigned 125 N 2 + lines and determined the molecular parameters for the (2, 1) band from these assignments. The analysis of the N 2 ∗ spectrum is ongoing and will be presented in a follow-up paper. The N 2 + (2, 1) band transition frequencies predicted from available literature values vary significantly from our assignments. The observed deviations indicate correlation of other parameters with the spin-rotation parameter and the quartic centrifugal distortion constants. These results indicate that rovibronic predictions for new N 2 + bands based on the parameters derived from individual band assignments are likely to be unreliable, and further investigation of the Meinel system may require a global analysis that accounts for this correlation.