Frequency measurement of pure rotational transitions of D₂O using tunable terahertz spectrometer

Precise frequency measurement of the rotational lines of D₂O in the terahertz (far-infrared) region is demonstrated as a case study of a high-precision far-infrared spectroscopy using a tunable radiation source. Frequencies of about 150 rotational lines of D₂O were measured in the 0.5 - 5 THz region. Measured frequencies provide an excellent frequency standard for the terahertz region together with our measurements on H ₂¹⁶O (v=0 and v₂=1 vibrational states), H ₂¹⁷O, and H₂¹⁸O. Molecular parameters of Watson's A-reduced Hamiltonian have been obtained to reproduce the observed frequencies.