Selected lines of CH₃CN, HCN, and HC₃N observed in the plasma of a flowing mixture of CH₄ and N₂ with a total pressure of 32.4 × 10⁻³ mbar. The integration time was 40 min in each W sub-band. The observing procedure was frequency switching with a throw of 37 MHz. The four rotational transitions of the ground state of HC₃N within the W band are shown in the middle panels. The different components of the J = 12 → 11 rotational transition of HC₃N in its vibrationally excited states ν₆, ν₇, and 2ν₇ are shown in the bottom panels.