Laser induced fluorescence spectroscopy of the jet-cooled SiNSi radical: Vibrational analysis of the XΠg2 state

We have generated SiNSi in supersonic free expansions and observed laser induced fluorescence (LIF) in the ultraviolet (UV) and visible regions. We measured two kinds of LIF excitation spectra from the X̃Πg2 electronic state: one detecting UV fluorescence and the other detecting visible fluorescence. By exciting the vibronic bands observed in the LIF excitation spectra, we recorded dispersed fluorescence (DF) spectra from the single vibronic levels to the X̃Πg2 state. The vibrational structures of the DF spectra are complicated due to the relatively large vibronic interactions in the X̃Πg2 state. We were not able to analyze the structure using regular formulations derived from perturbational treatments that consider the Renner-Teller (R-T) and spin-orbit (SO) couplings and the anharmonicity of the ν2 bending potential. Instead, we analyzed it by directly diagonalizing the vibronic Hamiltonian, including not only R-T, SO, and the anharmonicity but also Fermi resonance between the ν1 symmetric stretching and ν2 modes and anharmonicities, x11 and x12. The vibronic analysis revealed two characteristics: a relatively large R-T parameter, ϵ = +0.51, and a SO constant close to the bending frequency, A ≈ 0.8ω2. Both these characteristics bring about accidental proximity of the vibronic levels in the D∞h system, and this proximity causes heavy mixing among the vibronic levels. The results can be applied to find weak vibrationally hot bands in the rotationally resolved LIF excitation spectra observed, and actually the Q1p band head of the hot band, C̃(0110)Πg2 - X̃(0110)κΣg(+)2, has been able to be assigned. The band head assignment suggests an accuracy of a few cm-1 for the present vibronic analysis of the DF spectra, with a peak accuracy of ±3 cm-1.