The Far-Infrared Absorption Spectrum of HD16O: Experimental Line Positions, Accurate Empirical Energy Levels, and a Recommended Line List

The far-infrared absorption spectrum of monodeuterated water vapor, HD¹⁶O, is analyzed using three high-sensitivity absorption spectra recorded by high-resolution Fourier transform spectroscopy at the SOLEIL synchrotron facility. The gas sample was obtained using a 1:1 mixture of H₂O and D₂O leading to a HDO abundance close to 50%. The room temperature spectra recorded in the 50-720 cm^-1 range cover most of the rotational band. The sensitivity of the recordings allows for lowering by three orders of magnitude the detectivity threshold of previous absorption studies in the region. Line centers are determined with a typical accuracy of 5 × 10^-5 cm^-1 for well-isolated lines. The combined line list of 8522 water lines is assigned to 9186 transitions of the nine stable water isotopologues (H₂^XO, HD^XO, and D₂^XO with X = 16, 17, and 18). Regarding the HD¹⁶O isotopologue, a total of 2443 transitions are presently assigned while about 530 absorption transitions were available prior to our SOLEIL recordings. The comparison with the HITRAN list of HD¹⁶O transitions is discussed in detail. The obtained set of accurate HD¹⁶O transition frequencies is merged with literature sources to generate a set of 1121 accurate empirical rotation-vibration energies for the first five vibrational states (000), (010), (100), (020), and (001). The comparison to the previous dataset from an IUPAC task group illustrates a gain in the average energy accuracy by more than one order of magnitude. Based on these levels, a recommended list of transitions between the first five vibrational states is proposed for HD¹⁶O in the 0-4650 cm^-1 frequency range.