At the transition from the gas to the liquid phase of water, a wealth of new phenomena emerge, which are absent for isolated H 2 O molecules. Many of those are important for the existence of life, for astrophysics and atmospheric science. In particular, the response to electronic excitation changes completely as more degrees of freedom become available. Here we report the direct observation of an ultrafast transfer of energy across the hydrogen bridge in (H 2 O) 2 (a so-called water dimer). This intermolecular coulombic decay leads to an ejection of a low-energy electron from the molecular neighbour of the initially excited molecule. We observe that this decay is faster than the proton transfer that is usually a prominent pathway in the case of electronic excitation of small water clusters and leads to dissociation of the water dimer into two H 2 O , the observed decay channel might contribute as a source of electrons that can cause radiation damage in biological matter.The water molecule is, as a triatomic molecule, rather simple in structure and its geometry is well known. In contrast to that, the interplay of compounds of water molecules or other atoms and molecules with water, for example in a solution, is very rich and far from being fully understood. At the very onset of condensation when two water molecules are combined to form a water dimer a new dimension of complexity arises: electronic excitation of this complex spawns nuclear dynamics leading to fragmentation into a protonated fragment (that is, H 3 O + ) and an OH group 3,4 . For this fragmentation, first a proton migrates from one of the molecules to its neighbour, usually along a distance that is larger than the bond lengths found in the water molecule itself. Such fragmentation dynamics are characteristic for larger clusters, as well 5 . Typical mass spectra of fragments of water droplets show a break-up into protonated cluster fragments (H 2 O) n H + of different sizes and into OH groups. A reason for this is the absence of direct transitions within the Franck-Condon region to break-up channels that do not involve proton migration [6][7][8] . Furthermore, the migration itself is highly efficient and occurs on a timescale of <60 fs (ref. 9).The response of condensed water to electronic excitation has far-reaching consequences for biological systems. Radiation damage to cells naturally depends sensitively on the routes by which energy deposited into the cells is finally distributed and which fragmentation and de-excitation pathways are favoured. Experiments have shown that the constituents of DNA are highly vulnerable to low-energy electrons 1 . These studies revealed that not only does primary ionization by high-energy particles or photons cause damage, but also that low-energy electrons in particular break-up biomolecules efficiently 2 . ). The red oval shows an internuclear distance of 2.9 Å with a corresponding KER of 4.9 eV after the photo reaction. b,c, The process observed in this experiment: an electron from the inner valence shell of one...

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...

We Coulomb explode argon and neon dimers, trimers, and tetramers by multiple ionization in an ultrashort 800 nm laser pulse. By measuring all momentum vectors of the singly charged ions in coincidence, we determine the ground state nuclear wave function of the dimer, trimer, and tetramer. Furthermore we retrieve the bond angles of the trimer in position space by applying a classical numerical simulation. For the argon and neon trimer, we find a structure close to the equilateral triangle. The width of the distribution around the equilateral triangle is considerably wider for neon than for argon.

We have measured the two-site double ionization of argon dimers by ultrashort laser pulses leading to fragmentation into two singly charged argon ions. Contrary to the expectations from a pure Coulomb explosion following rapid removal of one electron from each of the atoms, we find three distinct peaks in the kinetic energy release (KER) distribution. By measuring the angular distribution of the fragment ions and the vector momentum of one of the emitted electrons for circular and linear laser polarization, we are able to unravel the ionization mechanisms leading to the three features in the KER. The most abundant one results from tunnel ionization at one site followed by charge-enhanced tunnel ionization of the second atom. The second mechanism, which leads to a higher KER we identify as sequential tunnel ionization of both atoms accompanied by excitation. The third mechanism is present with linearly polarized light only. It is most likely a frustrated triple ionization, where the third electron does not escape but is trapped in a Rydberg state.

Efficiency and beam quality enhancement of high-power lasers require effective thermal management. In order to determine the temperature distribution in double-cladding high-power continuous wave fibre lasers, the heat transfer equation is analytically solved. Using the exact derived temperature distributions, the ultimate pump power before thermal damage, which is one of the key subjects of consideration in high-power laser design, is achieved.