Velocity map imaging spectroscopy of C₂H^- and C₂D^-: A benchmark study of vibronic coupling interactions

High-resolution velocity-map imaged photoelectron spectra of the ethynyl anions C₂H^- and C₂D^- are measured at photon wavelengths between 355 and 266 nm to investigate the complex interactions between the closely lying X̃²Σ⁺ and Ã²Π electronic states. An indicative kinetic energy resolution of 0.4%, together with the full angular dependence of the fast electrons, provides a detailed description of the vibronically coupled structure. It is demonstrated that a modest quadratic vibronic coupling model, parameterized by the quasidiabatic ansatz, is sufficient to accurately recreate all the observed vibronic interactions. Simulated spectra are shown to be in excellent agreement with the experimental data, verifying the proposed model and providing a framework that may be used to accurately simulate spectra of larger C_2nH monohydride carbon chains. New spectral assignments are supported by experimental electron anisotropy measurements and Dyson orbital calculations.