Sub-MHz accuracy measurement of the S(2) 2–0 transition frequency of D2 by Comb-Assisted Cavity Ring Down spectroscopy

The line position of the very weak S(2) transition of deuterium in the 2–0 band has been measured with a Comb-Assisted Cavity Ring Down spectrometer. The high sensitivity spectra were recorded at 5 and 10 mbar with a Noise Equivalent Absorption, α min , of 8 × 10 –11 cm −1 . The line positions at 5 and 10 mbar were measured with sub-MHz accuracy (460 and 260 kHz, respectively). After correction of the line pressure-shift, the frequency at zero pressure of the S(2) transition of the first overtone band was determined to be 187 104 299.51 ± 0.50 MHz. This value agrees within 1.7 MHz with the frequency obtained from the best available ab initio calculations and corresponds to only 15% of the claimed theoretical uncertainty.