Microwave spectrum of the $H2DO+$ ion: Inversion-rotation transitions and inversion splitting

Inversion-rotation spectral lines of the monodeuterated hydronium ion, $H2DO+$⁠, have been observed by a source-modulation spectrometer in the millimeter- to submillimeter-wave region. The ion was generated by a hollow-cathode discharge in a gas mixture of $H2O$ and $D2O$⁠. Nine inversion-rotation lines were measured precisely for the lowest pair of inversion doublets in the frequency region from $210to720GHz$⁠. The measured lines were analyzed to derive rotational constants in the inversion-doublet states and inversion splitting. The inversion splitting in the ground state was determined to be $1215866(410)MHz$⁠, that is, $40.5569(137)cm−1$⁠, where the numbers in parentheses give probable uncertainties estimated from the Jacobian matrix of the assumed centrifugal distortion constants of the inversion-doublet states. The determined inversion splitting is off by $−0.58cm−1$ from the predicted value of $41.14cm−1$ by Rayamäki et al using high-order coupled cluster ab initio calculations [J. Chem. Phys. 118, 10929 (2003)], and by $0.039cm−1$ from the observed value of $40.518(10)cm−1$ by Dong and Nesbitt using high-resolution jet-cooled infrared spectroscopy [J. Chem. Phys. 125, 144311 (2006)] beyond the quoted uncertainty. The most astronomically important transition $000−-10+$ for the ortho species was measured at $673257.024(31)MHz$⁠, which could be used as a radioastronomical probe investigating interstellar chemistry of deuterium fractionation in space.