Precision Sub-Doppler Millimeter and Submillimeter Lamb-Dip Spectrometer

We describe a precision sub-Doppler millimeter and submillimeter-wave Lamb-dip spectrometer with a backward-wave oscillator as the radiation source. The effect of nonlinear saturation of the spectral transitions (the Lamb-dip method) is used. The spectrometer resolution (about 5–10 kHz) and the measurement accuracy of the absolute frequencies (of the order of 1 kHz) of molecular transitions in the frequency range below 0.5 THz are discussed. The spectrometer is designed for obtaining accurate radio-astronomy and molecular-spectroscopy experimental data, in particular, when seeking variation in the proton-to-electron mass ratio as a function of time and place in the Universe. The frequency records of the Lamb dips on the spectral lines of the CO, OCS, and H 2 O molecules, the results of measuring the center frequencies of some transitions, and comparison with the results of other works are presented. The high measurement accuracy allows us to use the molecular-transition frequencies as the secondary frequency standards.