Extending frequency metrology to increasingly complex molecules: SI-traceable sub-Doppler mid-IR spectroscopy of trioxane

TL;DR

This paper demonstrates sub-Doppler mid-infrared spectroscopy of the largest molecule to date, using a high-resolution spectrometer calibrated against primary standards, achieving unprecedented measurement precision for complex polyatomic molecules.

Contribution

It extends frequency metrology techniques to complex molecules like trioxane, enabling ultra-precise spectroscopic measurements with uncertainties as low as 5 kHz.

Findings

Achieved ~100 kHz resolution in mid-IR spectroscopy of trioxane.

Measured hundreds of rovibrational transition frequencies with ~5 kHz uncertainty.

Validated the extension of frequency metrology to larger polyatomic molecules.

Abstract

Bringing increasingly complex polyatomic molecules within reach of precision measurement experiments offers fascinating and far-reaching prospects ranging from Earth sciences and astrophysics, to metrology and quantum sciences. Here, we demonstrate sub-Doppler spectroscopic measurements in the mid-IR fingerprint region of, to our knowledge, the largest molecule to date. To this end, we use a high-resolution ~10.3 $\mu$m spectrometer based on a sub-Hz quantum cascade laser remotely calibrated against state-of-the-art primary frequency standards via a metrology-grade fibre link. We perform saturated absorption spectroscopy in the v5 CO stretching mode of 1,3,5-trioxane, (H2CO)3, at a resolution of ~100 kHz, allowing us to measure the absolute frequency of hundreds of rovibrational transitions at unprecedented uncertainties for such a complex species, as low as ~5 kHz. Our work demonstrates the extension of frequency metrology methodologies to ever larger molecular system, confirming the potential of the technologies we develop for bringing increasingly complex species within reach of ultra-precise measurement experiments.