Microwave spectroscopy of the HCCS and DCCS radicals (X̃²Π_i) in excited vibronic states: A study of the Renner-Teller effect

The microwave spectra of the HCCS and DCCS radicals are studied in the frequency range of 160-400 GHz and the rotational transition series are assigned to several low-lying vibronic states in the CCS or H(D)CC bending vibration. Analysis is carried out to obtain effective constants for respective vibronic states. The γ_eff constants for the vibronic μ/κ²Σ states are found to be anomalous, in that the variation of the γ_eff constants in the same bending mode is large up to 3 GHz and the γ_eff value can reach to nearly twice the rotational constants B_v. This behavior cannot be understood by the current Renner-Teller theory. We have developed a theory to include cross vibronic interaction between two vibronic ²Σ(v_t = 1) states in different bending modes. Since the difference of the vibrational quantum numbers for these states is Δ(v₄ + v₅) = 0, the interaction has a much larger effect than the one considered by Petelin and Kiselev [Int. J. Quantum Chem. 6, 701 (1972)] for the vibronic states with Δ(v₄ + u₅) = ± 2. Calculation with the newly derived expressions for γ_eff reproduces the anomaly in HCCS when the Renner parameters are fixed at ε₄=-0.37 and ε₅=+0.10 from the ab initio calculation, and the parameter |ε₄₅| for the cross vibronic interaction is varied to be 0.4, a value which is obtained for the first time. The relative sign of the above ε₄ and ε₅ values is explicitly judged to be correct. In addition, the B_eff and the P-doubling constants in the 2Π_i and ²Δ_i states are found to be effected by a higher-order perturbation of the cross vibronic interaction.