Renner–Teller vibronic analysis for a tetra-atomic molecule. II. The ground state of the HCCS free radical

We have studied the tetra-atomic Renner–Teller effect in the ground state of the jet-cooled HCCS and DCCS free radicals. The reactive intermediates were produced in a pulsed discharge jet using thiophene or deuterated thiophene precursors and the ground-state vibronic energies were measured using the single vibronic level emission spectroscopy technique. In HCCS, emission and fluorescence depletion experiments proved that some previously unassigned laser-induced fluorescence (LIF) features were anomalous $Δ3∕22−Π3∕22$ transitions, which gave information on several ground-state $Δ3∕22$ levels. In DCCS, rotational mixing of the excited state $v′=0(Π3∕22)$ level with the $41(1Σ1∕22)$ level allowed several ground-state $Σ2$ levels to be observed in emission. In addition, we have shown that some previously unassigned weak LIF bands of DCCS are anomalous $Π1∕22−Π3∕22$ transitions which have allowed us to accurately measure the ground-state spin-orbit splitting $(259.0cm−1)$ for the first time. All of the expected ground-state $Π3∕22$ levels and the majority of the $Π1∕22$ levels up to $1700cm−1$ have been observed for both isotopomers. The data have been fitted by an effective Hamiltonian matrix treatment that included Renner–Teller, spin-orbit coupling, anharmonicity, and Fermi resonance effects for the two bending vibrations and the CS stretching mode. This has allowed an unusually complete description of the orbital angular momentum coupling effects, including the determination of the $ϵ4$⁠, $ϵ5$⁠, and $ϵ45$ terms for the first time. Subtle effects in both the LIF and emission spectra signal the presence of Sears resonances in both the ground and excited states of these radicals.