High sensitivity CW-CRDS of 18O enriched water near 1.6 μm

The absorption spectrum of 18O enriched water has been recorded by continuous wave cavity ring down spectroscopy between 5905.7 and 6725.7 cm−1 using a series of fibred DFB lasers. The investigated spectral region corresponds to the important 1.55 μm transparency window of the atmosphere where water absorption is very weak. The typical CRDS sensitivity (noise equivalent absorption of 5×10−10 cm−1) allowed for the detection of lines with intensity as low as 10−28 cm/molecule while the minimum intensity value provided by HITRAN in the considered spectral region is 1.7×10−24 cm/molecule. The line parameters were retrieved with the help of an interactive least squares multi-lines fitting program assuming a Voigt function as line profile. Overall, 4510 absorption lines belonging to the H218O, H216O, HD18O, HD16O and H217O water isotopologues were measured. Their intensities range between 3×10−29 and 5×10−23 cm/molecule at 296 K and the typical accuracy on the line positions is 1×10−3 cm−1. 2074 of the observed lines attributed to H218O, HD18O and H217O are reported for the first time. The transitions were assigned on the basis of variational calculations resulting in 288, 135 and 38 newly determined rovibrational energy levels for the H218O, HD18O and H217O isotopologues, respectively. The new data set includes the band origin of the 4ν2 bending overtone of H218O at 6110.4239 cm−1 and rovibrational levels corresponding to J and Ka values up to 18 and 12, respectively, for the strongest bands of H218O: 4ν2, ν1+2ν2, 2ν2+ν3, 2ν1, ν1+ν3, and ν2+ν3. The obtained experimental results have been compared to the spectroscopic parameters provided by the HITRAN database and to the recent IUPAC critical review of the rovibrational spectrum of H218O and H217O as well as to variational calculations. Large discrepancies between the 4ν2 variationally predicted and experimental intensities have been evidenced for the H218O and H216O molecules.