UNRAVELING THE COMPLEX NEAR-UV SPECTRUM OF Si$_2$C

Abstract

Author Institution: Harvard-Smithsonian Center for Astrophysics, Cambridge, MA; 02138, and School of Engineering & Applied Sciences, Harvard; University, 29 Oxford St., Cambridge, MA 02138; IRAP; Universite de Toulouse, UPS; CNRS; 9 Av. colonel Roche, BP 44346, F-31028 Toulouse cedex 4; France; Department of Chemistry, Virginia Tech, Blacksburg, VA 24061Though it has yet to be detected in space, there is good reason to believe that the disilicon carbide molecule, Si$_2$C, is an abundant constituent of circumstellar shells such as IRC+10216, in which the isovalent ionic ring SiC$_2$ is highly conspicuous. Si$_2$C exhibits a surprisingly complicated R2C2PI spectrum, which EOM-CCSD calculations indicate arises from a bent-to-linear transition involving several interacting electronic states in the region of vertical excitation. Some of the observed spectral features show well-resolved emission spectra when probed by LIF/DF, yielding vibrational frequencies in excellent agreement with \emph{ab initio} and matrix IR studies of the ground electronic state. Partially resolved rotational structure for these bands, which appear to be dominated by a vibronically allowed transition to a dark $A_2$ state, suggests a ground state bond angle significantly larger than predicted by theory. Owing to the presence of overlapping, unresolved bands, in addition to spectral contamination from Si$_2$ and SiC, further experimental characterization would appear ideally suited to a two-dimensional fluorescence approach