The Radio Spectrum of Si3

The silicon trimer Si₃ and several of its rare isotopic species have been detected in the laboratory, and the frequencies of 66 rotational transitions in the range 5-42 GHz have been measured to a few parts in 10⁷. The most intense radio lines can be calculated from the derived spectroscopic constants to better than 3 km s^-1 in equivalent radial velocity up to frequencies of 50 GHz, adequate for a deep search in the best astronomical sources; above that frequency the uncertainties increase to of the order of 20 km s^-1 at 160 GHz, which is still sufficient for a deep search in a wide-line source such as IRC +10216.