  Second electron affinities of Au_N and Ag_N clusters and the dissociation
energies for fission of the Au_N^{2-} and Ag_N^{2-} dianions are calculated
using the finite-temperature shell-correction method and allowing for triaxial
deformations. Dianionic clusters with N > 2 are found to be energetically
stable against fission, leaving electron autodetachment as the dominant decay
process. The second electron affinities exhibit pronounced shell effects in
excellent agreement with measured abundance spectra for Au_N^{2-} (N < 30),
with appearance sizes N=12 for gold dianionic clusters and N=24 for silver
dianionic clusters.
