Terahertz spectroscopy of ground state HD¹⁸O

Terahertz absorption spectroscopy was employed to measure the ground state pure rotational transitions of the water isotopologue HD180. A total of 105 pure rotational transitions were observed in the 0.5–5.0 THz region with ∼100 kHz accuracy for the first time. The observed positions were fit to experimental accuracy using the Euler series expansion of the asymmetric-top Hamiltonian together with the literature Microwave, Far-IR and IR data in the ground state and ν₂. The new measurements and predictions reported here support the analysis of astronomical observations by high-resolution spectroscopic telescopes such as SOFIA and ALMA where laboratory rest frequencies with uncertainties of 1 MHz or less are required for proper analysis of velocity resolved astrophysical data.