  Highly excited C$_2{}^{-}$ ions prominently feature electron detachment at a
mean decay time near 3 milliseconds with hitherto unexplained origin.
Considering various sources of unimolecular decay, we attribute the signal to
the electronic C$^4\Sigma^+_u$ state. Quartet C$_2{}^{-}$ levels are found to
be stabilized against autodetachment by high rotation. Time constants of their
rotationally assisted autodetachment into levels opening energetically at lower
rotation are calculated by a theory based on the non-local resonance model. For
some final levels of significantly less rotation the results conclusively
explain the puzzling observations.
