THE ROTATIONAL SPECTRA OF (SiC2)-Si-29 AND (SiC2)-Si-30

The rotational spectra of (SiC2)-Si-29 and (SiC2)-Si-30, two silicon isotopic species of the abundant astronomical ring (SiC2)-Si-28, have been characterized in the millimeter band between 140 and 360 GHz in a low pressure discharge through SiH4, C2H2, and Ar. Precise rotational and centrifugal distortion constants have been derived for both species by fitting a standard asymmetric top Hamiltonian to 38 a-type transitions of (SiC2)-Si-29 and 35 of (SiC2)-Si-30; the data sets include transitions up to K-a = 8 and at least J = 16. With these new measurements in hand, the most intense radio transitions of both species either have been measured or can now be predicted to better than 1 km s(-1) in equivalent radial velocity up to 500 GHz, more than adequate accuracy for spectral line identifications in circumstellar shells of evolved carbon stars such as IRC+10216 where SiC2 is conspicuous. More than 10 new lines of (SiC2)-Si-29 and (SiC2)-Si-30 have been identified between 295 and 354 GHz in the interferometric spectral line survey of IRC+10216 with the Submillimeter Array