The absorption spectrum of water vapor in the 1.25 μm atmospheric window (7911–8337 cm−1)

Abstract

The absorption spectrum of water vapor in “natural” isotopic abundance has been recorded at room temperature by high sensitivity Continuous Wave Cavity Ring Down Spectroscopy (CW-CRDS) between 7911 and 8337cm−1. The investigated region covers most of the 1.25µm transparency window of importance for atmospheric applications. The recordings were performed with sensitivity on the order of αmin~2×10–11cm−1, more than two orders of magnitude better than previous investigations by Fourier Transform Spectroscopy (FTS). Measured line intensities cover a range of seven orders of magnitude (3×10–30–2×10–23cm/molecule at room temperature). The experimental line list provided as Supplementary Material includes more than 5000 transitions. As a result of the achieved sensitivity, more than 1150 lines of the experimental list were identified as being due to ammonia present as an impurity at the 5ppm concentration level in the water sample. Although incomplete, the obtained ammonia line list seems to be the first one in the region. More than 3193 water lines were assigned to 3560 transitions of five water isotopologues (H216O, H218O, H217O, HD16O and HD18O). The assignments were performed using known experimental energy levels and calculated spectra based on variational calculations by Schwenke and Partridge. The obtained results are compared to the most relevant previous studies by Fourier Transform Spectroscopy in the region and to the exhaustive review of rovibrational line positions and levels performed recently by an IUPAC sponsored task group. Two-hundred and sixty-six levels are newly determined and 46 are corrected by more than 0.015cm−1 compared to those recommended by the water IUPAC task group. The overall agreement between variational and measured intensities is satisfactory.A complete empirical list of 4473 transitions incorporating all the experimental information at disposal was constructed for water in the studied region. The intensity cut-off was fixed to 1×10–29cm/molecule at 296K. A detailed comparison with the line list as provided by the HITRAN database illustrates the advantages of the new list.