Continuous-wave cavity ringdown spectroscopy of the N2+ Meinel system (2, 1) band

The A2Πu-X2Σg+ system of N2+ was first observed in auroral emissions by Meinel in 1950. Although the N2+ 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 N2+ in a positive column discharge cell. Many lines of the first positive band system of N2∗ were also identified during the process of assigning this spectrum. The relative intensities of the N2∗ and N2+ 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 N2+ lines and determined the molecular parameters for the (2, 1) band from these assignments. The analysis of the N2∗ spectrum is ongoing and will be presented in a follow-up paper. The N2+ (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 N2+ 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.