Frequency comb-referenced cavity ring-down spectroscopy of natural water between 8041 and 8633 cm−1

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The 1.25 µm atmospheric transparency window is of importance for a number of atmospheric applications. As a continuation of our previous works on the improvement of water vapor line parameters in the near infrared, the room temperature absorption spectrum of water vapor in natural isotopic abundance is recorded with unprecedented sensitivity between 8041 and 8633 cm−1, using comb-referenced cavity ring-down spectroscopy. The line positions and intensities of more than 5400 lines were retrieved. Their intensities range between 3.6 × 10−30 and 1.5 × 10−22 cm/molecule. The high sensitivity and low noise level of the recordings (αmin≈ 10−11 cm−1) allow for measuring more than 1600 new lines and determine their positions with an accuracy of about 10−4 cm−1 in the case of isolated features. The rovibrational assignments were performed using known experimental energy levels and calculated spectra based on variational calculations by Schwenke and Partridge. The final line list is assigned to more than 5400 transitions of the first six water isotopologues (H216O, H218O, H217O, HD16O, HD18O and HD17O). The measured line positions allow to determine the energy of 79 new levels of H216O, H218O, H217O, and HD16O, and to correct 139 previously reported term values. Although a good agreement is generally observed, the comparison to the HITRAN2020 spectroscopic database and to the W2020 transition frequencies reveals a number of discrepancies both for line positions and line intensities. The lack of traceability of some HITRAN line parameters and some biases in the derivation procedure of the W2020 energy levels are confirmed in the studied range. Validation tests of the theoretical values of the line intensities against measured values show both band-by-band variations of the deviations on the order of a few % and line-by-line fluctuations within a given band.