Spectral line lists of a nitrogen gas discharge for wavelength calibration in the range $4500-11000$cm$^{-1}$

Abstract

A discharge of nitrogen gas, as created in a microwave-induced plasma, exhibits a very dense molecular emission line spectrum. Emission spectra of this kind could serve as wavelength calibrators for high-resolution astrophysical spectrographs in the near-infrared, where only very few calibration sources are currently available. The compilation of a spectral line list and the characterization of line intensities and line density belong to the initial steps when investigating the feasibility of potential wavelength calibration sources. Although the molecular nitrogen spectrum was extensively studied in the past, to our knowledge, no line list exists that covers a continuous range of several thousand wavenumbers in the near-infrared. We recorded three high-resolution ($\Delta \tilde{\nu} = 0.018$cm$^{-1}$) spectra of a nitrogen gas discharge operated at different microwave powers. The nitrogen gas is kept inside a sealed glass cell at a pressure of 2mbar. The emission lines in the spectra were fitted by a superposition of Gaussian profiles to determine their position, relative intensity, and width. The line parameters were corrected for an absolute wavelength scale, instrumental line broadening, and intensity modulation. Molecular and atomic transitions of nitrogen were identified with available line positions from the literature. We report line lists with more than 40000 emission lines in the spectral range $4500-11000$cm$^{-1}$ ($0.9-2.2$$\mu$m). The spectra exhibit emission lines over the complete spectral range under investigation with about $350-1300$ lines per 100cm$^{-1}$. Depending on the microwave power, a fraction of $35\% - 55\%$ of all lines are blended. The total dynamic range of the detected lines covers about four orders of magnitude.