Experimental Separation of Virtual Photon Exchange and Electron Transfer in Interatomic Coulombic Decay of Neon Dimers

We investigate the interatomic Coulombic decay (ICD) of neon dimers following photoionization with simultaneous excitation of the ionized atom (shakeup) in a multiparticle coincidence experiment. We find that, depending on the parity of the excited state, which determines whether ICD takes place via virtual dipole photon emission or overlap of the wave functions, the decay happens at different internuclear distances, illustrating that nuclear dynamics heavily influence the electronic decay in the neon dimer.

Figure 1

(a) Two possible paths of decay in a neon dimer after creation of a $2s$ hole (adapted from 6). Left pathway: a virtual photon is transferred between the two atoms of the dimer, leading to the emission of a $2p$ electron of the right atom. Right pathway: competing process in which an electron is transferred leading to the emission of a $2p$. (b) Corresponding process after population of $2p$ shakeup states, which is investigated in the present experiment.

Figure 2

(a) The measured electron energy $E_e$ and kinetic energy of the ions KER on a logarithmic color scale. Clearly visible are the features corresponding to ICD after $2s$ ionization: the peak of $2s$ photoelectron (A) and the diagonal line belonging to the detected ICD electrons (B). Furthermore, some features at higher KERs and different photo electron energies are visible. (b) Total measured kinetic energy ($E_{e1}+E_{e2}+\mathrm{KER}$) of the four particle breakup of the dimer in dependence of the KER. The graph shows that for all events shown in (a), the sum kinetic energy is, within the experiment’s resolution, 15.6 eV, thus showing that the photo ions are not electronically excited in their final state and no energy was lost, e.g., by emission of a photon during the process.

Figure 3

The distribution of shakeup lines from 23 (top) in comparison with our results (bottom). Our results show the electron energy in dependence of the KER; the KER-distribution itself is shown on the left. The internuclear distance according to Eq. (4) is depicted on the right. Pairs of features belonging to a shakeup state and its corresponding second electron from interatomic decay are labeled with the same letters followed by 1 (for the shakeup line) or “2” (for the corresponding second electron). Shakeup states with even parity appear close to the mean internuclear distance of the dimer’s ground state of $R=3.3\AA$. Shakeup states of odd parity appear at a higher $\mathrm{KER}>5.5\mathrm{eV}$, i.e., at smaller internuclear distances.