Communication: Molecular near-infrared transitions determined with sub-kHz accuracy

Precise molecular transition frequencies are needed in various studies including the test of fundamental physics. Two well isolated ro-vibrational transitions of ¹²C¹⁶O at 1.57 μm, R(9) and R(10) in the second overtone band, were measured by a comb-locked cavity ring-down spectrometer. Despite the weakness of the lines (Einstein coefficient A≃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 191 360 212 761.1 and 191 440 612 662.2 kHz, respectively, with an uncertainty of 0.5 kHz (δν/ν=2.6×10^-12). The present work demonstrates the possibility to explore extensive molecular lines in the near-infrared with sub-kHz accuracy.