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 10mbar with a Noise Equivalent Absorption, αmin , of 8×10-11 cm-1. The line positions at 5 and 10mbar were measured with sub-MHz accuracy (460 and 260kHz, 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 187104299.51±0.50MHz. This value agrees within 1.7MHz with the frequency obtained from the best available ab initio calculations and corresponds to only 15% of the claimed theoretical uncertainty.