title: Communication: The ground electronic state of Si2C: Rovibrational level structure, quantum monodromy, and astrophysical implications.
abstract: We report the gas-phase optical detection of Si2C near 390 nm and the first experimental investigation of the rovibrational structure of its (1)A1 ground electronic state using mass-resolved and fluorescence spectroscopy and variational calculations performed on a high-level ab initio potential. From this joint study, it is possible to assign all observed Ka = 1 vibrational levels up to 3800 cm(-1) with confidence, as well as a number of levels in the Ka = 0, 2,  and 3 manifolds. Dixon-dip plots for the bending coordinate (ν2) allow an experimental determination of a barrier to linearity of 783(48) cm(-1) (2σ), in good agreement with theory (802(9) cm(-1)). The calculated (Ka, ν2) eigenvalue lattice shows an archetypal example of quantum monodromy (absence of a globally valid set of quantum numbers) that is reflected by the experimentally observed rovibrational levels. The present study provides a solid foundation for infrared and optical surveys of Si2C in astronomical objects, particularly in the photosphere of N- and J-type carbon stars where the isovalent SiC2 molecule is known to be abundant.
authors.affiliation: Harvard-Smithsonian Center for Astrophysics, 60 Garden St., Cambridge, Massachusetts 02138, USA.
authors.forename: Neil J
authors.initials: NJ
authors.lastname: Reilly
authors.affiliation: JILA, National Institute of Standards and Technology and Department of Physics, University of Colorado, Boulder, Colorado 80309, USA.
authors.forename: P Bryan
authors.initials: PB
authors.lastname: Changala
authors.affiliation: Department of Chemistry, University of Colorado, Boulder, Colorado 80309, USA.
authors.forename: Joshua H
authors.initials: JH
authors.lastname: Baraban
authors.affiliation: Harvard-Smithsonian Center for Astrophysics, 60 Garden St., Cambridge, Massachusetts 02138, USA.
authors.forename: Damian L
authors.initials: DL
authors.lastname: Kokkin
authors.affiliation: Department of Chemistry, The University of Texas at Austin, Austin, Texas 78712, USA.
authors.forename: John F
authors.initials: JF
authors.lastname: Stanton
authors.affiliation: Harvard-Smithsonian Center for Astrophysics, 60 Garden St., Cambridge, Massachusetts 02138, USA.
authors.forename: Michael C
authors.initials: MC
authors.lastname: McCarthy