Visible spectroscopy of jet-cooled SiC2: Geometry and electronic structure

Abstract

SiC2 has been prepared in a supersonic molecular beam by laser vaporization of a silicon carbide rod within a pulsed supersonic nozzle. Rotational analysis of the 0-0 band of the well-known 4980 Å band system of this molecule reveals that, contrary to previous assumptions, the molecule is triangular in both the ground and excited electronic states. In both states the molecule is of C2V symmetry with a C–Si–C angle between 40° and 41°. The correct assignment of the spectrum is à ′B2←x̃ ′A1. The carbon–carbon bond length is 1.25 Å in the ground state, suggesting that the molecule is best understood as a silicon atom bound to the side of a triply bonded C2 fragment. The optical transition moment is polarized along the b axis of the molecule which is parallel to the carbon–carbon bond axis. In the à 1B2 excited state the carbon–carbon bond opens up to 1.30 Å consistent with a π*←π excitation of the carbon–carbon triple bond. The silicon–cargon distance is measured to be 1.81 Å in the x̃ 1A1 state, lengthening to 1.88 Å in the à ′B2 excited state. In addition, the ionization potential of SiC2 was determined to lie between 8.91 and 10.38 eV.