Experimental Proof of Strong Π-Σ Mixing in the Renner-Teller and Pseudo-Jahn-Teller Affected CCH+ (3Π) Ion.
Journal Article
Overview
abstract
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.
