FTMW spectra for c-C₅H. Coloured lines show spectra using argon (magenta) and neon (black) as the carrier gas in the supersonic expansion. Each line is split into two Doppler components because the direction of the supersonic jet expansion is parallel to the standing wave in the Fabry–Pérot cavity of the spectrometer. The difference in the magnitude of Doppler splitting when different carrier gases – neon and argon – are used is related to the velocity of the molecule in the jet; it is reciprocal to the square root of the atomic mass of the carrier gas.