15min:

D. L. KOKKIN, IRAP; Université de Toulouse, UPS; CNRS; 9 Av. colonel Roche, BP 44346, F-31028 Toulouse cedex 4, France and Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138, and School of Engineering & Applied Sciences, Harvard University, 29 Oxford St., Cambridge, MA 02138; N. J. REILLY AND M. C. MCCARTHY, Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138, and School of Engineering & Applied Sciences, Harvard University, 29 Oxford St., Cambridge, MA 02138; R. C. FORTENBERRY AND T. D. CRAWFORD, Department of Chemistry, Virginia Tech, Blacksburg, VA 24061.

The gas phase optical spectra of the silicon terminated carbon chains, SiC_nH (n=3-5) formed in a silane acetylene discharge, have been investigated by R2C2PI and LIF/DF and will be reported here for the first time. Complementary to the experimental work, a theoretical investigation was undertaken with coupled cluster methods to garner a comprehensive understanding of the molecular structures and electronic properties of these systems. For the linear chains where there is an odd number of carbon atoms (SiC₃H and SiC₅H), the observed transitions are primarily from a ² ground state to a ² state, but as in the case of isovalent carbon chains there are some Herzberg-Teller active modes from an excited ² state. While a strong - transition is predicted for SiC₄H, the spectrum is dominated by relatively dark sigma state which is vibronically coupled to the bright ² state. In contrast to the odd carbon chains, which exhibit relatively sharp spectral features and lifetimes in the 10-100 ns regime, SiC₄H shows broadened spectral features consistent with a ca. 10 ps lifetime, and a subsequent long-lived decay (>30 microseconds) which we tentatively interpret in terms of mixing with a nearby quartet state arising from the same electronic configuration, a process unavailable for the odd chains.