Broadband and highly sensitive comb-assisted cavity ring down spectroscopy of CO near 1.57 μm with sub-MHz frequency accuracy

A self-referenced frequency comb has been combined with a cavity ring down (CRD) spectrometer to achieve a sub-MHz accuracy on the derived positions of the absorption lines. The frequency emitted by the distributed feedback (DFB) laser diode used in the spectrometer was obtained from the frequency of its beat note with the closest mode of the frequency comb. This delivers excellent frequency accuracy over a broad spectral region with sensitivity (noise equivalent absorption) of 1 x 10(-11) cm(-1) Hz(-1/2). This setup is used to measure the absorption spectrum of CO over a wide range corresponding to the 3-0 band (6172.5-6418.0 cm(-1)). Accurate values of line centers are measured for a total of 184 lines of four CO isotopologues, namely (CO)-C-12-O-60, (CO)-C-13-O-16, (CO)-C-12-O-18 and (CO)-C-12-O-17 present in "natural" abundances in our sample. The measurements include the first extensive study of the 3-0 band of (CO)-C-12-O-18 and (CO)-C-12-O-17, of the 4-1 hot band of (CO)-C-12-O-16 and the detection of new high-J transitions of the 3-0 band of (CO)-C-12-O-16 up to J=34. The line centers were corrected for the self-pressure shift and used to derive the upper state spectroscopic parameters. The obtained standard deviation of about 300 kHz and 500 kHz for the 3-0 band of (CO)-C-12-O-16 and of the minor isotopologues, respectively, is a good estimate of the average accuracy of the reported line centers. The resulting 3-0 line list of (CO)-C-12-O-16 provided as Supplementary material includes 69 reference line positions with a 300 kHz accuracy for the 6183-6418 cm(-1) region. (C) 2014 Elsevier Ltd. All rights reserved.