Observed emission of the J = 4−3 OCS rotational line for different experimental conditions. The doublet of the ν₂J = 4−3 transition is also detected in the experiments. The top spectrum was taken in ON/OFF mode, that is, signal through OCS filled cell minus signal through empty gas cell. The other spectra were acquired in FSW mode. The blanked channels in the baseline correspond to the negative features produced by the frequency folding of the spectra. The insert shows the J = 4−3 line of OC³⁴S at 47 462.353 MHz for P(OCS) = 0.5 mbar.