Astrophysics and many other fields require detailed knowledge and accurate understanding of the rotation-vibration spectrum of water isotopologues. The water isotopologue H216O plays an important role in the Earth’s climate as a greenhouse gas in the atmosphere. Furtenbacher et al. assembled the most complete database to date of experimentally measured rovibrational transitions of H216O.

The authors extended an IUPAC database, last updated in 2013, by adding 78 more sources and roughly 100,000 additional entries of transitions, bringing the total number to almost 300,000. The authors also used an improved methodology to shape the collated data. Using network analysis tools they developed, and taking into account updated quantum-chemical results, they were able to identify questionable measurements and inverted the measured transitions to energy levels.

W2020 contains nearly 20,000 rovibrational energy levels with statistically significant uncertainties and a total coverage for the energies of the quantum states of H216O beyond 9000 cm−1. The 20,000 empirical energy levels represent the measured 300,000 transitions and could facilitate further experiments to yield more energy levels. According to their estimation, the total number of rovibrational energy levels of H216O is close to one million and most are still unknown.

“Our study defines a new level for water spectroscopy,” said author Attila G. Császár. “It is also important to emphasize that the techniques leading to the improved results are general and can be applied to other gas-phase molecules of chemical, physical, and astronomical interest.”

Next, the authors are planning to use the same technique to build similar datasets for other water isotopologues, such as HD16O and H218O, which are also atmospheric gases of note.

Source: “W2020: A database of validated rovibrational experimental transitions and empirical energy levels of H216O,” by Tibor Furtenbacher, Roland Tóbiás, Jonathan Tennyson, Oleg L. Polyansky, and Attila G. Császár, Journal of Physical and Chemical Reference Data (2020). The article can be accessed at