Spectral fit of the 2_-1,2–1_-1,1 E (grey lines), 2_0,2–1_0,1 A (magenta), and 2_0,2–1_0,1 E (blue) triplets as observed towards B1b using the NOEMA narrow-band spectrometer. The spectra are centred at the frequency of the 2_0,2–1_0,1 A transition: 96 741.38 MHz. The transitions producing the emission lines (see Table 1) are reported, with the corresponding frequencies marked by small vertical red lines. The dashed vertical line is for the systemic velocity: +2.6 km s⁻¹ (e.g. Bachiller et al. 2001). Each transition shows a main peak at +0.5 km s⁻¹ plus a secondary peak further blueshifted by ~5 km s⁻¹.