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Spontaneous electron emission from hot silver dimer anions: Breakdown of the Born-Oppenheimer approximation

We report the first experimental evidence of spontaneous electron emission from a homonuclear dimer anion through direct measurements of $\rm{Ag}_2^- \rightarrow \rm{Ag}_2 + \rm{e}^-$ decays on milliseconds and seconds time scales. This observation is very surprising as there is no avoided crossing between adiabatic energy curves to mediate such a process. The process is weak but yet dominates the decay signal after 100 ms when ensembles of internally hot Ag$_2^-$ ions are stored in the cryogenic ion-beam storage ring, DESIREE, for 10 seconds. The electron emission process is associated with an instantaneous, very large, reduction of the vibrational energy of the dimer system. This represents a dramatic deviation from a Born-Oppenheimer description of dimer dynamics.