High-resolution laser spectroscopy of the linear SiC2Si molecule and its astrophysical implications

Abstract

Small silicon-carbon clusters are important gas-phase constituents of stellar atmospheres, and are thought to play a role as potential seeds of the interstellar dusts formed in the envelopes of evolved carbon stars. Here we present the high-resolution optical spectra of the linear SiC2Si molecule (l-Si2C2) studied via laboratory experiments. The l-Si2C2 molecules are generated in a supersonically expanding planar plasma by discharging a silane-acetylene-argon gas mixture. The optical absorption spectra in the 5000−5300 Å region are recorded using sensitive pulsed cavity ring-down spectroscopy. In total, five optical absorption bands belonging to the $ \tilde{C} ^{3}\Sigma_{u} ^{-} $ – $ \tilde{X} ^{3}\Sigma_{g} ^{-} $ electronic transition system of l-Si2C2 are measured with fully resolved spin splitting fine structures in individual rotational transitions. Accurate spectroscopic constants for both $ \tilde{X} ^{3}\Sigma_{g} ^{-} $ and $ \tilde{C} ^{3}\Sigma_{u} ^{-} $ states of l-Si2C2, including the spin-spin interaction constants and spin-rotation interaction constants, are determined from the experimental spectra, which can be used to simulate these optical bands with different temperatures. Using the determined spectroscopic constants, optical spectra of l-Si2C2 simulated with different rotational excitation temperatures are compared to the stellar spectra of evolved carbon stars V Hya and IRAS 12311−23509, where the triatomic SiC2 are known to be abundant. Tentative assignments of the l-Si2C2 spectral features in the stellar spectra are discussed.