  On the millisecond to second time scale, stored beams of diatomic carbon
anions C$_2{}^-$ 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 C$_2{}^-$, 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 C$_2{}^-$ 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 C$_2{}^-$ 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
C$_2{}^-$ ions in quasi-stable levels of the quartet electronic states are the
so far unidentified source of unimolecular decay.
