Laboratory spectrum of the HSCO⁺J_Ka, Kc = 4_1, 4 − 3_0, 3 rotational transition around 318 GHz, acquired with the free-unit jet experiment. The integration time is ∼15 min with 30 μs time constant. The red dashed line represents the best fit to a speed-dependent Voigt profile, as discussed in the text. Each rotational transition has a double-peaked line shape, the result of the Doppler shift of the supersonic molecular beam relative to the two travelling waves that compose the radiation beam.