  Calculations are presented which use the molecular R-matrix with
pseudo-states (MRMPS) method to treat electron impact electron detachment and
electronic excitation of the carbon dimer anion. Resonances are found above the
ionisation threshold of C$_2^-$ with $^1\Sigma^+_g$, $^1\Pi_g$ and $^3\Pi_g$
symmetry. These are shape resonances trapped by the effect of an attractive
polarisation potential competing with a repulsive Coulomb interaction. The
$\Pi_g$ resonances are found to give structure in the detachment cross section
similar to that observed experimentally. Both excitation and detachment cross
sections are found to be dominated by large impact parameter collisions whose
contribution is modelled using the Born approximation.
