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 ${\text{Ag}}_{2}^{\ensuremath{-}}\ensuremath{\rightarrow}{\text{Ag}}_{2}+{e}^{\ensuremath{-}}$ decays on milliseconds and seconds timescales. This observation is very surprising as there is no avoided crossing between adiabatic energy curves to mediate such a process. The process is weak, yet dominates the decay signal after 100 ms when ensembles of internally hot ${\mathrm{Ag}}_{2}^{\ensuremath{-}}$ ions are stored in the cryogenic ion-beam storage ring, DESIREE, for 10 s. 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.