High resolution emission FT spectra of sodium in a microwave discharge: Intensity variation of the D[formula omitted]/D[formula omitted] lines in exoplanetary atmospheres

Abstract

The intensity ratio of the D1 and D2 sodium-doublet emission lines varies both across spectra of astronomical sources and in laboratory plasmala. This article probes the behaviour of this ratio under controlled laboratory conditions. In order to study this variation, we have recorded microwave discharge spectra of sodium vapour in range of 15000 to 18000 cm−1. Emission bands of the molecular nitrogen buffer gas and kinetic broadening of nitrogen and sodium lines have been used in a characterisation of the discharge temperature. We show that the relative intensity of doublet lines is determined by self-absorption and depends on the buffer-gas pressure and discharge power. The ratio and line shapes can be explained by a two-temperature model of sodium emission and foreground absorption. Line ratios estimated for spectra with various strength of the self-absorption of electric discharges can be used for characterising their temperatures and densities. In contemporary astronomical observation of exoplanetary atmospheres, this effect has not been resolved yet. To observe this effect, at least 0.01 - 0.05 cm−1 resolution (resolving power 1 600 000) is required. We make a connection to stellar Na emission and possible absorption processes for future observations of exoplanetary atmospheres.