Abstract
There is an increasing demand for precise molecular spectroscopy, in particular in the mid-infrared fingerprint window that hosts a considerable number of vibrational signatures, whether it be for modeling our atmosphere, interpreting astrophysical spectra or testing fundamental physics. We present a high-resolution mid-infrared spectrometer traceable to primary frequency standards. It combines a widely tunable ultra-narrow Quantum Cascade Laser (QCL), an optical frequency comb and a compact multipass cell. The QCL frequency is stabilized onto a comb controlled with a remote near-infrared ultra-stable laser, transferred through a fiber link. The resulting QCL frequency stability is below 10-15 from 0.1 to 10s and its frequency uncertainty of 4x10-14 is given by the remote frequency standards. Continuous tuning over ~400 MHz is reported. We use the apparatus to perform saturated absorption spectroscopy of methanol in the low-pressure multipass cell and demonstrate a statistical uncertainty at the kHz level on transition center frequencies, confirming its potential for driving the next generation technology required for precise spectroscopic measurements.
Highlights
Precise spectroscopic analysis of various molecular systems enables many exciting advances in physical chemistry and fundamental physics
We report the development of a fully operational widely tunable SI-traceable MIR spectrometer based on a quantum cascade laser (QCL) stabilized to a remote NIR ultra-stable reference transferred from the French national metrological institutes (NMI) LNE-SYRTE to Laboratoire de Physique des Lasers (LPL) with an optical fiber link
We carry out saturated absorption spectroscopy from 970 to 973 cm-1, taking full advantage of the QCL’s tunability, we report continuous tuning of the spectrometer over ~400 MHz at the precision of the frequency reference and demonstrate a record sub-10 kHz methanol resonance frequency uncertainty
Summary
Precise spectroscopic analysis of various molecular systems enables many exciting advances in physical chemistry and fundamental physics. One of the simplest asymmetric-top with a hindered internal rotor, methanol has a rather intricate rotation-torsion-vibration energy structure and is as such a very important molecule for fundamental infrared and microwave spectroscopy [31,32,33], metrological applications and frequency calibration [34,35], the realization of optically-pumped far-infrared gas lasers [36] or fundamental physics tests It has for instance been identified as a very good candidate for probing the limits of the Standard Model because it is one of the most sensitive molecules for a search of a varying proton-to-electron mass ratio [37]. Spectroscopic measurement at this level of precision in the MIR are scarce, and to our knowledge, there exist only one single other methanol frequency measurement of a weak absorption line around 947.7 cm-1 with an uncertainty of 2.4 kHz comparable to ours [34]
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