Communication: The ground electronic state of Si₂C: Rovibrational level structure, quantum monodromy, and astrophysical implications

We report the gas-phase optical detection of Si₂C near 390 nm and the first experimental investigation of the rovibrational structure of its ¹A₁ 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 K_a = 1 vibrational levels up to 3800 cm^-1 with confidence, as well as a number of levels in the K_a = 0, 2, and 3 manifolds. Dixon-dip plots for the bending coordinate (v₂) 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 (K_a, v₂) 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.