High sensitivity ICLAS of D2O between 12 450 and 12 850 cm−1

The weak absorption spectrum of dideuterated water, D2O, has been recorded between 12 450 and 12 850 cm−1 by high sensitivity Intracavity Laser Absorption Spectroscopy (ICLAS). This spectral region corresponds to the (ν1 + ν2/2 + ν3) = 5 polyad, dominated by the 4ν1 + ν3 band centered at 12 743.035 cm−1. The achieved sensitivity has allowed for the detection of lines with a minimum intensity of 2 × 10−28 cm/molecule i.e. typically two orders of magnitude lower than previous observations in the region considered. A total of 586 energy levels belonging to 11 vibrational states were determined. The rovibrational assignment process of 1025 lines ascribed to D2O was based on new results of variational calculations by Shirin et al. [S.V. Shirin, N.F. Zobov, O.L. Polyansky, J. Quant. Spectrosc. Radiat. Transfer, in press, doi:10.1016/j.jqsrt.2007.07.010]. The overall agreement between these calculations and the observed spectrum is good both for the line positions and line intensities. The difficulties encountered while performing the rovibrational labeling and the assignment of the weakest transitions not included in Combination Differences relations, are discussed.