We measured the spontaneous decays of internally hot copper and silver dimer anions, ${\mathrm{Cu}}_2^{-}$ and ${\mathrm{Ag}}_2^{-}$, stored in one of the two ion-beam storage rings of the Double Electrostatic Ion Ring Experiment (DESIREE) at Stockholm University. A coincidence detection technique was utilized enabling essentially background-free measurements of ${\mathrm{Cu}}_2^{-}\to$ Cu $+ {\mathrm{Cu}}^{-}$ and ${\mathrm{Ag}}_2^{-}\to$ Ag $+ {\mathrm{Ag}}^{-}$ fragmentation rates. Furthermore, the total rates of neutral decay products (monomers and dimers) were measured and the relative contributions of fragmentation and electron emission (${\mathrm{Cu}}_2^{-}\to {\mathrm{Cu}}_2 + e^{-}$ and ${\mathrm{Ag}}_2^{-}\to {\mathrm{Ag}}_2 + e^{-}$) were deduced as functions of storage time. Fragmentation is completely dominant at early times. However, after about 20 ms of storage, electron emission is observed and becomes the leading decay path after 100 ms for both dimer anions. The branching ratios between fragmentation and electron emission (vibrationally assisted autodetachment processes) are very nearly the same for ${\mathrm{Cu}}_2^{-}$ and ${\mathrm{Ag}}_2^{-}$ throughout the present storage cycle of 10 seconds. This is surprising considering the difference between the electron affinities of the neutral dimers, ${\mathrm{Cu}}_2$ and ${\mathrm{Ag}}_2$, and the difference between the ${\mathrm{Cu}}_2^{-}$ and the ${\mathrm{Ag}}_2^{-}$ dissociation energies.

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• Cryogenic technology
• Electronic excitation & ionization
• Potential energy surfaces
• Rotational states