Tunneling dynamics and spectroscopic parameters of monodeuterated hydronium, H2DO+, from a combined analysis of infrared and sub-millimeter spectra

Abstract

The infrared laser direct absorption spectrum of H₂DO⁺ in the OH stretching region was reported quite recently revealing large amplitude tunneling dynamics. The large rotational constants make the jet-cooled spectrum relatively sparse at low rotational temperatures and assignments thus challenging. Transitions were assigned through ground state combination differences, with additional tentative assignments made via comparison of predicted/observed spectra. More recently, 9 rotation-inversion transitions were recorded in the sub-millimeter (sub-mm) region, which yielded tunneling splittings and rotational constants differing slightly from IR results. This has prompted the present reinvestigation of the H₂DO⁺ spectra, which now takes full advantage of the combined data from both studies. While previous analyses considered each tunneling state as independent and non-interacting, the present analysis is based on a tunneling-Hamiltonian model for the well studied, isoelectronic NH₂D molecule, modified to account for the larger tunneling splitting. The combined analysis revealed rotational interaction between tunneling states as well as between the two OH stretching modes and permitted a substantial number of new assignments to be made, including one sub-millimeter transition while only few IR assignments had to be corrected or omitted. It leads to improvement in parameters for both the ground as well as for the OH stretching states of this important molecular ion, which reproduce the assigned lines within experimental uncertainties, provides guidance, e.g., for the spectral search in the OD stretch region, and yields deeper insight into the tunneling dynamics.