Experimental Proof of Strong Π–Σ Mixing in the Renner–Teller and Pseudo-Jahn–Teller Affected CCH ⁺ ( ³ Π) Ion

The ethynyl radical cation, CCH⁺ (³Π), offers a unique system for fundamental spectroscopic studies of nonadiabatic effects due to its open-shell linear structure and the presence of a low-lying ³Σ⁻ state, which induces notable perturbations in the (ro-)vibrational spectrum. To probe these effects, we recorded the broadband vibrational spectrum of CCH⁺ from 350-3450 cm⁻¹ using leak-out spectroscopy. The spectrum reveals a complex splitting pattern in the CCH bending mode attributed to Renner-Teller and pseudo-Jahn-Teller coupling effects between the ³Π and ³Σ⁻ electronic states. A three-state diabatic model, validated here against high-resolution IR data of the CH stretching mode, facilitated assignments within the broadband infrared (IR) spectrum, including an additional Π vibronic feature observed in the aforementioned high-resolution spectrum. Our results highlight a pronounced sensitivity of the splitting pattern to the Π-Σ energy gap, with couplings so large that even the zero-point vibrational motion of the bending vibration is sufficient to disrupt the vibronic structure of this ion. This compact ion, with strong coupling effects and high-quality spectroscopic data, serves as an exemplary system for evaluating nonadiabatic models.