Decay channels and appearance sizes of doubly anionic gold and silver clusters

Second electron affinities of Au-N and Ag-N clusters and the dissociation e nergies for fission of the Au-N(2-) and Ag-N(2-) dianions are calculated us ing the finite-temperature shell-correction method and allowing for triaxia l deformations. Dianionic clusters with N>2 are found to be energetically s table against fission, leaving electron autodetachment as the dominant deca y 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(a)(2-)(Au) = 12 and n(a)(2-)(Ag) = 24.