ECAMP 15

Terahertz (THz, 0.1-10 THz) precision spectroscopy of various molecular species is essential for several applications such as astrophysics and planetary atmosphere sensing, molecular physics, and tests of fundamental physics, yet it remains technically challenging. Such measurements require on one side the development of a dedicated spectrometer allowing for broadband measurements at ultra-high resolution (and for which the frequency axis is referenced to SI frequency standards) and on the other side to develop methodologies to probe the molecular transitions under sub-Doppler regime in order to resolve the finest intramolecular couplings. Both considerations are challenging in the THz range. In the present poster, we will present our current efforts to obtain THz absorption spectra of various molecules under sub-Doppler regime using a THz frequency multiplication chain. The frequency axis of our measurements is referenced to SI standard thanks to the recent reception of both RF and optical frequency references provided by the SYRTE laboratory and disseminated by the REFIMEVE+ network.
We tested two different approaches to record sub-Doppler spectra for non-reactive molecular species up to 1.5THz: double resonance (Mid-IR pump and THz probe) and saturated absorption experiments. These two techniques were used to measure the pure rotational transitions of ammonia (NH3) and methane (CH4). We will present the experimental set-up we developed and the spectroscopy results obtained for both molecules. In particular the observation of the finest coupling in NH3 (coupling of the H spins with the rotation angular momentum) allows us to determine the molecular structure in the ground and the first vibrationally excited state.