Two transitions of anti vinyl mercaptan recorded in the microwave (top trace, in green) and millimeter-wave (bottom trace, in blue) domains. The frequency axis is given as an offset from the rest frequencies, 22 346.6731 and 85 467.985 MHz, respectively. The doublet structure of the microwave transition (top spectrum) is caused by Doppler splitting, which arises because of the coaxial arrangement of the cavity axis with respect to the axis along which the gas expands; the rest frequency is taken as the average of the two components. The second derivative line shape aspect of the millimeter transition (bottom spectrum) arises from 2f demodulation of the modulated frequency.