The spectral lines of photons emitted from excited atoms, ions and molecules have been used ever
since their discovery as a Fingerprint of the particle's electronic structure. In 1925 Pierre Auger
discovered, that in competition to the emission of these characteristic photons, excited atoms can
release their energy by emission of one of their electrons in a process commonly known as "Auger
decay". Seven years ago, Cederbaum et al.  predicted a third, additional decay mechanism
termed Interatomic Coulombic Decay (ICD). This novel effect is calculated to become the
dominant decay channel in many cases, once the excited atom is placed in an environment of other atoms. In that
case the deexcitation energy is transferred to a neighboring atom, which releases it by emission of
its most loosely bound electron. Here we report on an experimental observation of Interatomic
Coulombic Decay in 2s ionized neon-dimers. The decay is unambiguously identified by detecting the
energy of two Ne1+ fragments and the ICD electron in coincidence, yielding a clean experimental
spectral distribution of the ICD electrons.
Fig.1: Sequence of events observed in the present experiment. (a) Creation of a 2s hole in a
neon dimer by photoionization (b) Successive Interatomic Coulombic Decay: the 2s hole is filled by a
2p electron, the excess energy is transferred to the neighboring neon atom causing the ejection of
one of its 2p electrons. (c) Back-to-back emission of the fragments after Coulomb explosion of the
In order to unambiguously identify ICD in a neon dimer experimentally the momenta of the emitted particles are measured in coincidence. As Fig.2 shows, the measured momenta of the two ions are of the same magnitude but opposite direction, just as expected for the event of a coulomb exploding dimer. Furthermore, for the case of ICD, the sum of the ICD electron's energy and the kinetic energy of the ionic fragments is a constant. Therefore by looking at the relation of the electron's energy and the kinetic energy of the ionic fragments (Fig.3) events of ICD appear on a diagonal line and are therefore unambiguosly identified.
Fig.2: Coincidently measured momenta of the two Ne1+ ions. The events are found along a diagonal line pz1 = -pz2, where pz1;2 is the momentum component of ion 1 and 2 parallel to the field of the spectrometer. This proves directly that
the two ions result from a Coulomb explosion of a dimer, i.e. it unambiguously identifies step
(c) in Figure 1.
Fig.3: Measured electron energy in dependence of the coincidently measured kinetic energy of the ionic fragments (KER). As the sum of both is a constant for the case of ICD, events of ICD cause a diagonal line in the plot.
The experiment was performed in a colaboration with the group of Prof. U. Hergenhahn (Max-Planck-Institut für Plasmaphysik) at the Berlin synchrotron facility BESSY II.
 L. S. Cederbaum, J. Zobeley, and F. Tarantelli. Phys. Rev. Lett., 79:4778, 1997.