Precision microwave spectroscopy in a Ku-band waveguide: The case study of the 12.2 GHz line of methanol
(2026) Journal of Quantitative Spectroscopy and Radiative Transfer — Vol. 353, p. 109840 (2026)
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- Authors
Hays, Brian 
- Abstract
- Accurate rest frequencies of torsion-rotation transitions of methanol in the centimeter wave are critical to challenge the standard model through probing hypothetical variation of fundamental constants over space and time. Even though microwave Fourier transform (FTMW) spectroscopy is a very mature technique, it fails to provide uncertainties below the kHz level and the characterization of these uncertainties remains scarce. Here, we employ a new FTMW spectrometer to measure and analyze the free induction decay (FID) signal of the 12.2 GHz torsion-rotation transition of methanol in the time domain. We discuss the systematic effects that induce a shift on the line center and quantify the associated corrections and uncertainties that pertain to the frequency estimate. The transition frequency was determined to be 12, 178, 596, 106 ± (12) stat ± (243) sys Hz. This work not only provides a reference to further constrain the limit on hypothetical variation of the proton-to-electron mass ratio, but also compiles the many systematic effects that must be accounted for in general to accurately quantify the uncertainty of the frequency estimated from FID signals.
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Citations
Collignon, S., Hays, B., Lederer, D., & Lauzin, C. (2026). Precision microwave spectroscopy in a Ku-band waveguide: The case study of the 12.2 GHz line of methanol. Journal of Quantitative Spectroscopy and Radiative Transfer, 353, 109840. https://doi.org/10.1016/j.jqsrt.2026.109840 (Original work published 2026)