Vibronic coupling in the X̃ ²Π and à ²Σ⁺ states of HCN⁺

The vibronic structure associated with the X̃ ²Π and à ²Σ⁺ states of HCN⁺ have been examined by high resolution threshold photoelectron spectroscopy using coherent vacuum ultraviolet radiation and pulsed field ionization techniques. Rotationally resolved spectra for 12 vibronic bands are presented from which vibronic symmetry assignments and cation spectroscopic constants are derived. The latter are compared to the ab initio calculations of Köppel et al. [Chem. Phys. 37, 303 (1979)] who calculated the vibronic structure of HCN ⁺ based on strong X̃-à interstate coupling. Finally, the observed rotational branch structure is used to infer the ionization dynamics of strongly coupled vibronic levels for which the Born-Oppenheimer approximation is not valid.