Four K_a = 0 a-type rotational transitions for the HC₃HCN radical. For the 2_0,2–1_0,1, 3_0,3 –2_0,2, and 4_0,4 –3_0,3 transitions, additional hyperfine components, which are not shown in this figure, were also observed. The abscissa corresponds to the frequency of the lines in MHz. The spectra were achieved by 100 shots of accumulation and a step scan of 0.5 MHz with a repetition rate of 10 Hz. The coaxial arrangement of the adiabatic expansion and the resonator axis produces an instrumental Doppler doubling. The resonance frequencies are calculated as the average of the two Doppler components.