Correlated Electron Dynamics at Surfaces Investigated via ${\mathrm{He}}^{2+}$ Ion Neutralization

The neutralization of a single ${\mathrm{He}}^{2+}$ ion near a Ir surface leads to the emission of an electron pair. Via coincidence spectroscopy we give evidence that a sizable amount of these electron pairs originate from a correlated single step neutralization of the ion involving a total of four electrons from the metal. These correlated electron pairs cannot be explained in the common picture of two consecutive and independent neutralization steps. We infer a characteristic time scale for the correlated electron dynamics in the metal of $40–400\mathrm{as}$.

Figure 1

Neutralization step via Auger capture. $E_F$ ($W$) denotes the Fermi level (work function) of the metal. The energy level of the projectile at infinite distance is $E_b$. The atomic levels are shifted close to the surface due to the image charge. A metal electron makes a transition into the lower lying level of the ion leading to electron emission from the metal.

Figure 2

Arrival time histogram of electron pairs emitted from Ir(100) due to 10 eV ${\mathrm{He}}^{2+}$ ions. The peak is indicative of “true” coincidences while the constant background arises from “random” coincidences. The dashed vertical lines indicate the selection for the energy spectra of Fig. 3. Excitation with 5 eV ${\mathrm{He}}^{+}$ ions showed no evidence of pair emission; see inset.

Figure 3

2D-energy spectra for a central energy of $E_k=19\mathrm{eV}$ (a) and $E_k=30\mathrm{eV}$ (b). The red lines mark an $L$-shaped region; events within this originate from a sequential emission. The dashed black diagonal lines mark $E_{\mathrm{sum}}^{\max }$. The pair of black solid lines in (b) label the region $|E_{\text{right}}-E_{\text{left}}|\le 8\mathrm{eV}$ used for the $E_{\mathrm{sum}}$ spectrum of Fig. 4.

Figure 4

Panel (a) shows the $E_{\mathrm{sum}}$ spectrum of the data presented in Fig. 3 under the constraint $|E_{\text{right}}-E_{\text{left}}|\le 8\mathrm{eV}$. The vertical dashed line marks $E_{\mathrm{sum}}^{\max }$. In (b) we plot the $\mathrm{\Delta }t$ histogram for an $E_{\mathrm{sum}}$ range just below or above $E_{\mathrm{sum}}^{\max }$ as indicated by the two boxes in (a).

Figure 5

In (a) a core hole is filled by an electron and pair emission called double Auger decay takes place [32]. In the three-electron Auger (b), a double vacancy is filled by an electron pair and the gained energy is transferred to a third electron [33, 34]. In (c) an electron pair of the metal fills the double vacancy and an electron pair from the metal is emitted.