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

Hefei National Laboratory for Physical Sciences at the Microscale, and Department of Chemical Physics, University of Science and Technology of China, Hefei, Anhui 230026, PR China
e-mail: dzhao@ustc.edu.cn

Received: 6 January 2020
Accepted: 28 March 2020

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 ${\stackrel{\sim }{C}}^3{\mathrm{\Sigma }}_u^{-}$ – ${\stackrel{\sim }{X}}^3{\mathrm{\Sigma }}_g^{-}$ electronic transition system of l-Si₂C₂ are measured with fully resolved spin splitting fine structures in individual rotational transitions. Accurate spectroscopic constants for both ${\stackrel{\sim }{X}}^3{\mathrm{\Sigma }}_g^{-}$ and ${\stackrel{\sim }{C}}^3{\mathrm{\Sigma }}_u^{-}$ 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.