Compilation of HI and ¹³CO(1–0) spectra for the central area of the G28.3 IRDC (averaged over the central 100″ squared). The thin black histogram shows the original HI emission data from the THOR survey (Beuther et al. 2016; Wang et al. 2020a) where the HI self-absorption (HISA) is visible as absorption against the typically bright ~100 K emission of the Galaxy. The red line presents a second-order polynomial fit to the non-HISA part of the spectrum. The red histogram then shows the resulting inverted HISA spectrum for this feature. The dashed histogram presents the ¹³CO(1–0) emission (Jackson et al. 2006) for the same region (multiplied by 20 for easier readability).