Precise Frequencies of H₂¹⁶O Lines Protected for Radio Astronomy

Precise frequency values have been determined for H₂¹⁶O radio lines appearing in protected line lists of the International Astronomical Union and the Panel on Frequency Allocations of the US National Academy of Sciences. The improved precision is attributable to a spectroscopic network built from a large set of near-infrared Lamb-dip lines augmented with a handful of ultra-high-accuracy rotational transitions. The ultraprecise H₂¹⁶O network contains 376 Lamb-dip lines recorded previously via our frequency-comb-locked cavity-enhanced spectrometer. During the present study, a total of 55 target lines have been (re)measured at high accuracy. Due to our network-assisted measurements, the accuracy has been significantly improved with respect to previous direct radio-frequency measurements for all the protected lines of H₂¹⁶O above 750 GHz. Furthermore, 43 of these protected transitions now benefit from the accuracy of the new near-infrared Lamb dips reported in this paper.