Unimolecular processes in diatomic carbon anions at high rotational excitation

On the millisecond to second time scale, stored beams of diatomic carbon anions C₂^- from a sputter ion source feature unimolecular decay of yet unexplained origin by electron emission and fragmentation. To account for the magnitude and time dependence of the experimental rates, levels with high rotational and vibrational excitation are modeled for the lowest electronic states of C₂^-, also including the lowest quartet potential. Energies, spontaneous radiative decay rates (including spin-forbidden quartet-level decay), and tunneling dissociation rates are determined for a large number of highly excited C₂^- levels and their population in sputter-type ion sources is considered. For the quartet levels, the stability against autodetachment is addressed and recently calculated rates of rotationally assisted autodetachment are applied. Non-adiabatic vibrational autodetachment rates of high vibrational levels in the doublet C₂^- ground potential are also calculated. The results are combined to model the experimental unimolecular decay signals. Comparison of the modeled to the experimental rates measured at the Croygenic Storage Ring (CSR) gives strong evidence that C₂^- ions in quasi-stable levels of the quartet electronic states are the so far unidentified source of unimolecular decay.