The radio spectra of SiCCH, SiCN, and SiNC

Three new silicon-bearing radicals of astrophysical interest, SiCCH and the two nearly isoenergetic isomers SiCN and SiNC, were detected in a laborato ry discharge in their (XII)-I-2 ground states by Fourier transform microwav e and millimeter-wave absorption spectroscopy. Hyperfine structure was obse rved in the low rotational transitions of the (II1/2)-I-2 ladder, and well- resolved Lambda-doubling was observed in both fine-structure ladders. With the spectroscopic constants derived from the laboratory measurements, the s pectra of all three can be calculated to an uncertainty of less than 0.1 km s(-1) in equivalent radial velocity over the entire range of interest to r adio astronomers. SiCN, with a dipole moment of 2.9 D, is probably the most promising of the three for astronomical discovery.