Absolute frequency determination of molecular transition in the Doppler regime at kHz level of accuracy

•We measured unperturbed frequency of Doppler-broadenedmolecular transition in O2 B band with relative uncertainty of 2×10−11. We obtained absolute transition frequency with uncertainty typical for Doppler-free techniques.•Spectra were measured with optical frequency comb (OFC) assisted, Pound-Drever-Hall (PDH) locked, frequency stabilized cavity ring-down spectrometer (FS-CRDS) linked to the optical atomic clock.•We present proof-of-principle experiment introducing the use of optical atomic clocks as a frequency reference in molecular spectroscopy.•We show that the use of optical atomic clock as a frequency reference together with self-referenced optical frequency comb can improve spectrum frequency axis even though the comb is not locked to the clock.

We measured absolute frequency of the unperturbed P7 P7 O2 B-band transition ν0= 434783.5084857(82) GHz and the collisional self-shift coefficient δ=−9.381(62)×10−21 GHz/(molecule/cm3). With Doppler-limited spectroscopy we achieved the relative standard uncertainty of 2×10−11 on line position, typical for Doppler-free techniques. Shapes of the spectral line were measured with a Pound-Drever-Hall-locked frequency-stabilized cavity ring-down spectrometer referenced to an 88Sr optical atomic clock via an optical frequency comb.