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...

The ion momentum spectra for nonsequential double ionization of rare gases (He, Ne, and Ar) in 23 fs 795-nm laser pulses were measured in the intensity range between 0.075 and 1.25 PW/cm 2 . Confusing differences in the shape of so far published momentum distributions between the different targets are consistently explained within a recollision scenario: Excitation during recollision plus subsequent field ionization, not implemented in most theoretical approaches, is provided evidence to play a decisive role for He and Ar nonsequential double ionization whereas it plays only a minor role for Ne.32.80. Rm, 31.90.+s , 32.80.Fb Many-electron dynamics in intense laser fields has been the subject of a large number of theoretical and experimental investigations. In particular, strong field double ionization is a fundamental process and, at the same time, one of the most challenging problems in atomic and optical physics, theoretically as well as experimentally. Whereas single ionization is well understood within the ësingle active electroní (SAE) model, the

We have measured the momentum distributions of singly and doubly charged helium ions created in the focus of 220 fs, 800 nm laser pulses at intensities of (2.9-6.6)x10(14) W/cm(2). All ions are emitted strongly aligned along the direction of polarization of the light. We find the typical momenta of the He2+ ions to be 5-10 times larger than those of the He1+ ions and a two peak structure at the highest intensity.

The dissociative electron transfer from He into 10 keV H2+ was measured in a kinematically complete experiment by using the cold target recoil ion momentum spectroscopy imaging technique in combination with a highly resolving molecular fragment imaging technique. The electron transfer into the dissociative b(3)Sigma+_(u) state of H2 could be selected by kinematic conditions. We find a striking double slit interference pattern in the transverse momentum transfer which we can modify by selecting different internuclear distances. Compared to an optical double slit, interference minima and maxima are interchanged. The latter is the result of a phase shift in the electronic part of the wave function.

We demonstrate experimentally the difference between a sequential interaction of a femtosecond laser field with two electrons and a nonsequential process of double ionization mediated by electron-electron correlation. This is possible by observing the momentum distribution of doubly charged argon ions created in the laser field. In the regime of laser intensities where the nonsequential process dominates, an increase in laser power leads to an increase in the observed ion momenta. At the onset of the sequential process, however, a higher laser power leads to colder ions. The momentum distributions of the ions from the sequential process can be modelled by convolving the single-ionization distribution with itself.