FTMW spectrum for the H₂CCCN radical showing the 4_0, 4-3_0, 3 rotational transition at 34.9 GHz. Much weaker 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 200 shots of accumulation and a step scan of 1 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.