W-band spectrum of a CH₄, N₂, and CH₃CN plasma at a total pressure of 28 × 10⁻³ mbar. The RF power is 35 W. The observing time per W sub-band is 26 min. Only the CH₃CN precursor shows rotational transitions, since N₂ and CH₄ have no permenent electric dipole moment. We note, however, the forest of lines appearing in this short integration time. The strong feature around 88 631 MHz corresponds to HCN. Other polar species produced in the plasma are HC₃N, CH₂CHCN, CH₃CH₂CN, and CN (this one is only observed in the optical spectrum). A zoom-in on the J = 1 → 0 line of HCN and its isotopologues is shown in Fig. 15. The frequency region around the J = 12 → 11 lines of HC₃N is shown in Fig. C.6.