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During the last 15 years a novel decay mechanism of excited atoms has been discovered and investigated. This so called "Interatomic Coulombic Decay" (ICD) involves the chemical environment of the electronically excited atom: the excitation energy is transferred (in many cases over long distances) to a neighbor of the initially excited particle usually ionizing that neighbor. It turned out that ICD is a very common decay route in nature as it occurs across van-der-Waals and hydrogen bonds. The time evolution of ICD is predicted to be highly complex, as its efficiency strongly depends on the distance of the atoms involved and this distance typically changes during the decay. Here we present the first direct measurement of the temporal evolution of ICD using a novel experimental approach.In 1997 Cederbaum and coworkers realized that the presence of loosely bound atomic or molecular neighbors opens a new relaxation pathway to an electronically excited atom or molecule. In the decay mechanism they proposed -termed Intermolecular Coulombic Decay (ICD) -the excited particle relaxes efficiently by transferring its excitation energy to a neighboring atom or molecule [1]. As a consequence the atom or molecule receiving the energy emits an electron of low kinetic energy. The occurrence of ICD was proven in experiments in the mid 2000s by means of electron spectroscopy [2] and multi-coincidence techniques [3]. Since that time a wealth of experimental and theoretical studies have shown that ICD is a rather common decay path in nature, as it occurs almost everywhere in loosely bound matter. It has been proven to occur after a manifold of initial excitation schemes such as innervalence shell ionization, after Auger cascades [4,5], resonant excitation [6,7], shakeup ionization [8] and resonant Auger decay. ICD has also been observed in many systems as rare gas clusters [9], even on surfaces [10] and small water droplets [11,12]. The latter suggested that ICD might play a role in radiation damage of living tissue [13], as it creates low energy electrons, which are known to be genotoxic [14,15]. More recently that scenario was reversed as it was suggested to employ ICD in treatment of tagged malignant cells [16]. Apart from these potential applications the elementary process of ICD is under investigation, as the decay is predicted to have a highly complex temporal * Electronic address: jahnke@atom.uni-frankfurt.de behavior. The efficiency and thus the decay times of ICD depend strongly on the size of the system, i.e. the number of neighboring particles and the distance between them and the excited particle. However, even for most simple possible model systems consisting of only two atoms the temporal evolution of the decay is non-trivial and predicted theoretically to exhibit exciting physics [17]: as ICD happens on a timescale that is fast compared to relaxation via photon emission, but comparable to the typical times of nuclear motion in the system, the dynamics of the decay is complicated and so far only theoretically explored...

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We show that a single photon can ionize the two helium atoms of the helium dimer in a distance up to 10 Å . The energy sharing among the electrons, the angular distributions of the ions and electrons, as well as comparison with electron impact data for helium atoms suggest a knockoff type double ionization process. The Coulomb explosion imaging of He 2 provides a direct view of the nuclear wave function of this by far most extended and most diffuse of all naturally existing molecules. DOI: 10.1103/PhysRevLett.104.153401 PACS numbers: 36.40.Àc, 34.80.Dp The helium dimer ( 4 He 2 ) is an outstanding example of a fragile molecule whose existence was disputed for a long time because of the very weak interaction potential [1] (see black curve in Fig. 1). Unequivocal experimental evidence for 4 He 2 was first provided in 1994 in diffraction experiments [2] by a nanostructured transmission grating. Subsequently, the average dimer bond length and dimer binding energy could be determined to be 52 Å and 10 À7 eV (0:9 Â 10 À3 cm À1 or 1.3 mK) [3]. This very large bond length, a factor 100 larger than the hydrogen bond length, goes along with a prediction of very widely delocalized wave function, unseen in any other molecule [4] (see R 2 É 2 i function in Fig. 1). It is because of these exotic properties that ''as the hydrogen molecule in the past, the helium dimer today became a test case for the development of new computational methods and tools '' [5] in quantum chemistry. Despite this fundamental nature of the diffuse helium dimer wave function, it has escaped direct experimental observation until now, as the diffraction grating experiment measures the mean value and not the shape of the wave function itself. Our experiment provides a direct view of this diffuse object.The large distance between the two helium atoms and the minuscule binding energy make the helium dimer a unique model system to explore electron correlations over large distances. The most sensitive tool for such studies is multiple photoionization. Since photoabsorption is described by a single electron operator the photon energy and angular momentum is best thought of as being initially given to one electron of the atom only. In the absence of electron correlation the ejection of a single electron would be the only possible outcome of the photoabsorption process. Because of the ubiquity of electron correlation, however, the ejection of electron pairs by a single photon is a wide spread phenomenon seen in atoms [6], molecules [7,8], and solids [9]. This two electron process poses at least two central questions: what is the correlation mechanism by which the photon energy is distributed among the two electrons and over which distance are such correlations active? In the present Letter we report the surprising observation that a single photon can lead to nonsequential ejection of two electrons from two atoms which are separated by many atomic radii and where the overlap of the electronic wave functions is negligible. By measuring the internuclear dist...