THE ROTATIONAL SPECTRA OF 29 SiC 2 AND 30 SiC 2

Abstract

The rotational spectra of {sup 29}SiC{sub 2} and {sup 30}SiC{sub 2}, two silicon isotopic species of the abundant astronomical ring {sup 28}SiC{sub 2}, have been characterized in the millimeter band between 140 and 360 GHz in a low pressure discharge through SiH{sub 4}, C{sub 2}H{sub 2}, 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 {sup 29}SiC{sub 2} and 35 of {sup 30}SiC{sub 2}; the data sets include transitions up to K{sub 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{sup -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 SiC{sub 2} is conspicuous. More than 10 new lines of {sup 29}SiC{sub 2} and {sup 30}SiC{sub 2} have been identified between 295 and 354 GHz in the interferometric spectral line survey of IRC+10216 with the Submillimeter Array.