Molecular near-infrared transitions determined with sub-kHz accuracy

Precise molecular transition frequencies are of great interest in the test of fundamental physics and also in various applications. Two well isolated ro-vibrational transitions of $^{12}$C$^{16}$O at 1.57 $\mu$m, R(9) and R(10) in the second overtone band molecule, were measured by a comb-locked cavity ring-down spectrometer. Despite the weakness of the lines (Einstein coefficient $A\simeq 0.008$ s$^{-1}$), Lamb-dip spectra were recorded with a signal-to-noise ratio over 1000, and the line positions were determined to be 191 360 212 763.7 and 191 440 612 664.8 kHz respectively, with an uncertainty of 0.5 kHz ($\delta\nu/\nu=2.6\times 10^{-12}$). The present work demonstrated the possibility to explore extensive molecular lines in the near-infrared with sub-kHz accuracy.