D 216O: ICLAS between 13 600 and 14 020 cm −1 and normal mode labeling of the vibrational states

Abstract

The high resolution absorption spectrum of dideuterated water, D 2 16O, has been recorded by Intracavity Laser Absorption Spectroscopy (ICLAS) in the 13 600–14 020 cm −1 spectral region which is the highest energy region reported so far for this water isotopologue. Because the HD 16O absorption is stronger by three orders of magnitude in the region under study, it was necessary to use high deuterium enrichment in order to minimize the HD 16O absorption lines overlapping the D 2 16O spectrum. With the high sensitivity achieved (noise equivalent absorption α min ∼10 −9 cm −1), transitions with line strengths on the order of 5 × 10 −28 cm molecule −1 could be detected. The spectrum analysis, based on recent variational calculations has provided a set of 177 new rovibrational energy levels belonging to six vibrational states. The most complete set of 53 vibrational energy levels of D 2 16O, including the three newly determined band origins, was constructed from an exhaustive review of the literature data. The fitting of the parameters of the vibrational effective Hamiltonian has allowed to reproduce the whole set of vibrational energies with an rms deviation of 0.055 cm −1. This simple model gave consistent vibrational labels of the D 2 16O states up to 18 000 cm −1. Above 15 000 cm −1, Fermi and Darling-Dennison resonance interaction were found to induce strong vibrational mixings of the wave functions in the normal mode basis, leading to ambiguous vibrational labeling.