Optically Derived Radio-Frequency Benchmark in Methanol: A Sub-kHz Reference for Astrophysical Tests of Fundamental Physics

Methanol radio lines observed in space provide sensitive probes of whether the proton-to-electron mass ratio has changed over cosmic time, but such tests require laboratory rest frequencies with very high accuracy. Here we determine the frequency of the astrophysically important 12.2 GHz 3−1E-20E transition of CH3OH by measuring near-infrared rovibrational transitions rather than the microwave line directly. Using wavelength-modulated NICE-OHMS locked to an ultra-stable optical frequency comb and referenced via a fiber link to a hydrogen-maser frequency standard, we measure Lamb-dip frequencies near 1.4 µm (216 THz) with 10 Hz statistical reproducibility and absolute uncertainties as low as 130 Hz. Pairs of optical transitions sharing common upper levels form a triangulation scheme that yields the ground-state rotational combination difference. We obtain 12 178 596.415(135) kHz, improving on earlier molecular-beam microwave spectroscopy by a factor of 20 and agreeing with a recent free-induction-decay measurement. This result establishes a sub-kHz laboratory benchmark for a key radio-astronomical methanol line and demonstrates that optical triangulation can be extended to non-chiral molecules with internal rotation.

Affiliations

UCLouvainSST/IMCN/NAPS - Nanoscopic Physics
VSLDutch Metrology Institute
Vrije Universiteit AmsterdamDepartment of Physics and Astronomy, LaserLab
University of OsloInstitute of Theoretical Astrophysics, the Faculty of Mathematics and Natural Sciences
CNRSLISA

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Cozijn, F., Altman, A., Bogomolov, A., Libert, A., Dierikx, E., Koelemeij, J., Collignon, S., Lankhaar, B., Kleiner, I., Lauzin, C., & Ubachs, W. (2026). Optically Derived Radio-Frequency Benchmark in Methanol: A Sub-kHz Reference for Astrophysical Tests of Fundamental Physics. Optica. Accepted/in-press. https://doi.org/10.1364/OPTICA.593388 (Original work published 2026)