number of investigations of the visible spectra of the Ångström [@Hakalla2016] and Herzberg bands [@Hakalla2014] for CO isotopologues. The discharge lamp undergoes a preparation phase depositing either $^{12}$C or $^{13}$C atomic isotopes onto the electrodes after which the oxygen containing isotope of choice is then introduced and a DC-plasma formed by applying a potential (650V) onto the anodes. In the discharge, specific isotopologues of CO are formed and maintained at a certain density and temperature (600$-$700K) for several hours, emitting visible radiation to be investigated by spectroscopic means.

The spectroscopic system, previously composed of an Ebert plane-grating spectrograph combined with a laterally scanned photo-multiplier tube, was upgraded for the present investigation with a Bruker-IFS 125 HR Fourier-transform spectrometer recording inteferograms in the double-sided acquisition mode. The CO discharge light source was focused on the entrance aperture of the FTS by a plano-convex quartz lens. The photomultiplier tube operated in integration pulse mode was used as an internal detector covering the 11000–25000 [cm$^{-1}$]{} spectral region. The instrumental resolution limit was set at 0.018 [cm$^{-1}$]{}. Higher resolution was not necessary, as the Doppler-dominated linewidth (FWHM) of the CO lines was about 0.06 [cm$^{-1}$]{}. A total of 126 scans were co-added and the 