Interatomic Coulombic Decay: The Mechanism for Rapid Deexcitation of Hollow Atoms

The impact of a highly charged ion onto a solid gives rise to charge exchange between the ion and target atoms, so that a slow ion gets neutralized in the vicinity of the surface. Using highly charged Ar and Xe ions and the surface-only material graphene as a target, we show that the neutralization and deexcitation of the ions proceeds on a sub-10 fs time scale. We further demonstrate that a multiple Interatomic Coulombic Decay (ICD) model can describe the observed ultrafast deexcitation. Other deexcitation mechanisms involving nonradiative decay and quasimolecular orbital formation during the impact are not important, as follows from the comparison of our experimental data with the results of first-principles calculations. Our method also enables the estimation of ICD rates directly.

Figure 1

(a) Above surface charge capture into highly excited Rydberg states in the ion. A hollow atom is formed. (b) At close distance to the target atoms the hollow atom quenches and releases its excitation energy by excitation or ionization of target electrons via ICD.

Figure 2

(a) Measured number of stabilized electrons for ${\mathrm{Xe}}^{30+}$ and ${\mathrm{Ar}}^{16+}$ ions. The interaction time $\tau$ is calculated from the ion velocity $v_{\mathrm{ion}}$ and the neutralization length (first capture distance, see the Supplemental Material [22]). (b) Using different kinetic energies and fitting the data with an exponential function a charge neutralization time constant ${\tau }_n$ can be deduced.

Figure 3

(a) Projectile and target coordinates. (b) Auger neutralization and exchange transition. (c) Atomic Auger deexcitation and resonant resupply of electrons. (d) Electron sidefeeding and molecular orbital formation. (e) ICD.

Figure 4

(a) All electronic states of Xe get strongly demoted as a function of charge state and higher principal quantum numbers $n$ become binding. If the Xe ion captures electrons into high Rydberg states inner shells become slightly promoted again (hollow ion or atom formation). (b) Electron binding energies for a quasimolecule of neutral xenon and carbon as a function of the interatomic distance.

Figure 5

Atomic Auger rates for three specific hollow atom configurations shown as a histogram with probability of occurrence.