Laser induced fluorescence spectroscopy of the jet-cooled SiNSi radical: Rotational analyses of the C2Δu−X2Πg transition and vibronic analysis in the C2Δu state

We have generated SiNSi in supersonic free jet expansions and measured laser induced fluorescence (LIF) spectra in the ultraviolet region. On the basis of rotational analyses, nine vibronic bands in the LIF spectra have been attributed to the C̃Δu2 - X̃Πg2 electronic transition of SiNSi. The 32 393 and 32 505 cm-1 subbands have been assigned to the two spin components of the 000 band. In addition, the C̃ (0220) μΣu(+)2 -, (0200) Δu2 -, (0220) κΣu(-)2 -, (1000) Δu2 -, (0420) μΣu(+)2 -, (040,40) μΔu2 -, (0620)μΣu(+)2 -, and (1620)μΣu(+)2 - X̃(0000)Πg2 bands have been identified. The experimental identification that the Σu(+)2 level is lower than Σu(-)2 suggests that the in-phase bending potential, labeled as V+, is the lower (or outer) one for the C̃Δu2 electronic state. The Δl = ±2 vibronic bands, such as C̃ (0220) Σu(+)2 - X̃(0000)Πg2, which are forbidden by the Δl = 0 selection rule, are induced by the (quadratic) Renner-Teller (R-T) interaction of the C̃Δu2 state with the ẼΣu+2 state, i.e., the vibronic transition moment is induced by the quadratic R-T (2RT) interaction of the C̃Δu2 state with the ẼΣu+2 state. The vibronic bands can also obtain their transition moments through their neighbors, which are the vibronic bands of the electronically forbidden D̃Σg+2 - X̃Πg2 transition. Based on the theory of the quaternary R-T (4RT) interaction, the vibrational structure of the C̃Δu2 state has been analyzed, and a local perturbation between the C̃(02l0) levels and their neighbor, D̃(0110)Πu2, has been identified. The observations suggest that, although the direct couplings between the C̃Δu2 and D̃Σg+2 states are negligible, their indirect couplings through the other states are effective.