LETTERS PUBLISHED ONLINE: 10 JANUARY 2010 | DOI: 10.1038/NPHYS1498 Ultrafast energy transfer between water molecules T. Jahnke 1 * , H. Sann 1 , T. Havermeier 1 , K. Kreidi 1 , C. Stuck 1 , M. Meckel 1 , M. Schöffler 2 , N. Neumann 1 , R. Wallauer 1 , S. Voss 1 , A. Czasch 1 , O. Jagutzki 1 , A. Malakzadeh 1 , F. Afaneh 3 , Th. Weber 2 , H. Schmidt-Böcking 1 and R. Dörner 1 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 + ions. As electrons of low energy (∼0.7-20 eV) have recently been found to efficiently break- up DNA constituents 1,2 , 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 6–8 . 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 . 1 Institut für Kernphysik, University of Frankfurt, Max-von-Laue-Str. 1, D-60438 Frankfurt, Germany, 2 Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA, 3 Physics Department, The Hashemite University, PO Box 150459, Zarqa 13115, Jordan. *e-mail: jahnke@atom.uni-frankfurt.de. a c b Photoelectron ICD electron H 2 O + H 2 O + 2.9 Å Figure 1 | Investigated species and process. a, Geometry of the water dimer (adapted from ref. 21). 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 of the molecules of the dimer is ejected by absorption of a photon (b) and then the energy released by de-excitation at this site is transferred to the neighbouring site from where a second, low-energy electron is emitted (c). Here, we report the observation that inner-valence-ionized water dimers fragment, contrary to the standard scenario described above. They relax ultrafast and directly, without a preceding migration of protons. Their de-excitation is observed to occur along with the emission of a low-energy electron that has— depending on the states involved—an energy less than 10 eV. That energy range coincides with the energy range relevant for radiation damage. The relaxation occurs through an intermolecular coulombic decay (ICD), a process first predicted by Cederbaum and co-workers 12 years ago 10 . ICD occurs when the excited particle is only loosely attached to neighbouring particles by for example, Van der Waals forces or hydrogen bonding. In such a scenario, an intermolecular decay involving the emission of an electron from a neighbouring partner of the initially excited particle may become the dominant channel for de-excitation. ICD is a highly efficient ionization mechanism and happens for species investigated so far on timescales less than 100 fs. Other intermolecular or interatomic NATURE PHYSICS | VOL 6 | FEBRUARY 2010 | www.nature.com/naturephysics 139 © 2010 Macmillan Publishers Limited. All rights reserved.