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 ${\text{Ag}}_2^{-}\to {\text{Ag}}_2+e^{-}$ 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 ${\mathrm{Ag}}_2^{-}$ 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.

Figure 1

Schematic of the electrostatic ion-beam storage ring used for the present measurements [15, 16]. A 10 keV ${\mathrm{Ag}}_2^{-}$ silver dimer anion bunch of about ${10}^5$ ions is injected to circulate in a closed orbit at a revolution frequency of 10.8 kHz. Neutral particles Ag and ${\mathrm{Ag}}_2$ from spontaneous fragmentation and from electron-detachment processes in the straight section closest to the injection port are detected by the ND. Atomic anions, ${\mathrm{Ag}}^{-}$, from fragmentation events in the opposite straight section are detected by the FD.

Figure 2

Data recorded with $\approx {10}^5$ stored 10 keV ${\mathrm{Ag}}_2^{-}$ anions. (Upper) Background corrected detector count rates as functions of time after the ion production in the Cs sputter ion source: •, neutral products recorded by the neutral detector (ND in Fig. 1); $+$, ionic ${\mathrm{Ag}}^{-}$ fragments recorded by the fragment detector (FD in Fig. 1). (Lower) Ratio between the ${\mathrm{Ag}}^{-}$ and ($\mathrm{Ag}+{\mathrm{Ag}}_2$) count rates as measured by the FD and ND detectors, respectively (left vertical axis). The vertical axis to the right has been corrected for the slightly different detection efficiencies of the two detectors (see text) and thus shows the branching ratio for fragmentation directly. The statistical errors are smaller than the symbols in both panels.

Figure 3

Approximate potential-energy curves based on literature values from [12, 22] for rotational angular momentum $L=0$ (left) and $L=300$ (right). Black curves, $L=0$ and $L=300$ dimer anion electronic ground state potentials; gray curves, equivalent electronic ground state potentials of the neutral dimer. The observed fragmentation is interpreted as due to tunneling through the centrifugal barrier for dimer anions in high angular-momentum levels as suggested in Ref. [20]. The observed electron-detachment signal is ascribed to dimer anions in high vibrational levels with too low angular momenta to fragment but sufficient vibrational energy to detach the electron by the weak coupling of vibrational and electronic degrees of freedom (see text). The vibrational levels indicated in the left panel are $v=110$ for the anion and $v=7$ for the neutral dimer. The metastable rovibrational level illustrating the tunneling fragmentation as an example in the right panel has $L=300$ and $v=88$.

Figure 4

A map of the rovibrational levels of ${\mathrm{Ag}}_2^{-}$ indicating for which combinations of rotational and vibrational excitations fragmentation and/or electron emission are energetically allowed. The dot and triangle markings indicate the $L$ and $v$ corresponding to the levels drawn in the left and right panels of Fig. 3, respectively.