Lines of NC₄NH⁺ observed toward TMC-1. Blanked channels correspond to negative artifacts resulting from the frequency switching observing mode. Unidentified lines are labeled as U. The red lines correspond to the line profiles calculated under LTE adopting a column density of 1.2 × 10¹⁰ cm⁻², a rotational temperature of 5.6 K, a line width of 0.60 km s⁻¹, and an emission size of 40″ in radius (see text).