The hyperfine structure of the inversion-rotation transition JK=10 ← 00 of NH3 investigated by Lamb-dip spectroscopy

The ground-state inversion-rotation transition of ammonia at 572.498 GHz has been investigated in the laboratory using a source-modulation microwave spectrometer equipped with a double-pass cell. The experiment has been carried out at a pressure lower than 0.5 mTorr, thus enabling to record a Lamb-dip spectrum showing a partial resolution of the magnetic hyperfine structure due to the three proton spins. The achieved high resolution proved to be comparable to that of molecular beam experiments. This measurement provides very precise rest frequencies of the J(K) = 1(0) <- 0(0) transition and of its hyperfine components, which fall in a spectral region covered by the HIFI instrument on board the Herschel satellite.