Optical spectra of the silicon-terminated carbon chain radicals SiCnH (n = 3,4,5)

The gas-phase optical spectra of three silicon-terminated carbon chain radicals, SiCnH (n = 3 - 5), formed in a jet-cooled discharge of silane and acetylene, have been investigated by resonant two-color two-photon ionization and laser-induced fluorescence/dispersed fluorescence. Analysis of the spectra was facilitated by calculations performed using equation-of-motion coupled cluster methods. For SiC3H and SiC5H, the observed transitions are well-described as excitations from a (2)Π ground state to a (2)Σ state, in which vibronic coupling, likely involving a higher-lying Π state with a very large predicted f-value (close to unity), is persistent. The lowest (2)Σ states of both species are characterized by a rare silicon triple bond, which was identified previously [T. C. Smith, H. Y. Li, D. J. Clouthier, C. T. Kingston, and A. J. Merer, J. Chem. Phys. 112, 3662 (2000)] in the lowest (2)Σ state of SiCH. Although a strong Π - Π transition is predicted for SiC4H, the observed spectrum near 505 nm more likely corresponds to excitation to a relatively dark Σ state which is vibronically coupled to a nearby Π state. In contrast to the chains with an odd number of carbon atoms, which exhibit relatively sharp spectral features and lifetimes in the 10-100 ns range, SiC4H shows intrinsically broadened spectral features consistent with a ∼100 fs lifetime, and a subsequent long-lived decay (>50 μs) which we ascribe to mixing with a nearby quartet state arising from the same electronic configuration. The spin-orbit coupling constants for both SiC3H and SiC5H radicals were determined to be approximately 64 cm(-1), similar to that of SiCH (69.8 cm(-1)), suggesting that the unpaired electron in these species is localized on the silicon atom. Motivated by the new optical work, the rotational spectrum of linear SiC3H was detected by cavity Fourier-transform microwave spectroscopy in the 13-34 GHz range. Each rotational transition from the [Formula: see text] ground state exhibits well-resolved Λ-doubling and hyperfine structure; the derived rotational constant of B = 2.605 GHz is in excellent agreement with our calculations.