CNR-INO researchers have finally succeeded in extending absolute frequency metrology to buffer-gas-cooled molecular samples.

By reducing both the internal and translational temperature of any species down to a few kelvins, the buffer-gas-cooling (BGC) technique has the potential to dramatically improve the quality of ro-vibrational molecular spectra, thus offering unique opportunities for transition frequency measurements with unprecedented accuracy. However, the difficulty in integrating metrological-grade spectroscopic tools into bulky cryogenic equipment has hitherto prevented from approaching the kHz level even in the best cases.

Here, we overcome this drawback by an original opto-mechanical scheme which, effectively coupling a Lamb-dip saturated-absorption cavity ring-down (SCAR) spectrometer to a BGC source, allows us to determine the absolute frequency of a given acetylene ro-vibrational transition at 1.5-micron wavelength with an overall fractional uncertainty as low as 6*10⁻¹². By improving the previous record with buffer-gas-cooled molecules by one order of magnitude, our approach paves the way for a number of ultra-precise low-temperature spectroscopic studies, aimed at both fundamental Physics tests and optimized laser cooling strategies.

These results were recently published in Nature Communications, and also selected for the Editors’ Highlights pages in the “Inorganic and physical chemistry” area.