On employing H216O, H217O, H218O, and D216O lines as frequency standards in the 15-170 cm−1 window

The protocol MARVEL, standing for measured active rotational-vibrational energy levels, is used to study high-accuracy measurements of rotational lines of four isotopologues of water, H216O, H217O, H218O, and D216O, obtained by spectroscopy in the far-infrared (FIR) region of 15-170 cm−1 by Matsushima et al. [Matsushima F, Odashima H, Iwasaki T, Tsunekawa S, Takagi K. Frequency measurement of pure rotational transitions of H2O from 0.5 to 5 THz. J Mol Struct 1995; 352/353, 371-8; Matsushima F, Nagase H, Nakauchi T, Odashima H, Takagi K. Frequency measurement of pure rotational transitions of H217O and H218O from 0.5 to 5 THz. J Mol Spectrosc 1999;193: 217-23; Matsushima F, Matsunaga M, Qian GY, Ohtaki Y, Wang RL, Takagi K. Frequency measurement of pure rotational transitions of D2O from 0.5 to 5 THz. J Mol Spectrosc 2001;206: 41-6; Matsushima F, Tomatsu N, Nagai T, Moriwaki Y, Takagi K. Frequency measurement of pure rotational transitions in the v2=1 state of H2O. J Mol Spectrosc 2006;235: 190-5]. MARVEL validates the high accuracy of most of the measured line positions. It results in a considerable number of energy levels with an average internal uncertainty of only 40 kHz (2σ). It also supports serious inaccuracy problems when Watson-type A-reduced Hamiltonians are used for predicting the highly accurate rotational measurements for water. Finally, MARVEL suggests a large number of para-water levels, for example 41 for H216O, which are candidates for becoming frequency standards in the FIR region of 15-170 cm−1 (the 0.5-5 THz window) when an accuracy of about 0.1 MHz is deemed to be sufficient.