Laboratory spectra of HSCO⁺: the J_Ka, Kc = 33_4, * − 30_4, * (overlapping) doublet around 371 GHz, acquired in 725 s integration time with 3 ms time constant; the strong spikes visible in the plot are due to instabilities of the DC discharge (top panel). The b-type J_Ka, Kc = 1_1, 0 − 1_0, 1 rotational transition around 274 GHz, acquired in double resonance with a 45 kHz step, and in a total integration time of 20 min (30 shots per integration step at 5 Hz) (bottom panel). The red dashed lines represent the best fit to a speed-dependent Voigt (top) and Gaussian profile (bottom). A schematic diagram indicating the two transitions that share a common energy level is shown in the inset.