Pure CO₂ 01¹ 0(1) → 00⁰0(1) emission spectrum around 15 μm, showing the central Q-branch and the P and R side branches. Left panel: full wavelength range is depicted; the grey line shows the original FLITS spectrum, the black line shows the results convolved with a R = 600 Gaussian, and the blue line the results convolved with a R = 3000 Gaussian. Right panel: zoom into the Q-branch band-head, plotting only the original FLITS spectrum. The individual lines have Keplerian double-peaked profiles, and merge with each other at maximum. At the band head, the total flux is smaller than expected from the sum of the individual Q-branch line fluxes because the lines physically overlap and partly shield each other in the disc.