The radio spectrum of Si3

The silicon trimer Si-3 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 10(7). 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.