Ultrafast energy transfer between water molecules

At the transition from the gas to the liquid phase of water, a wealth of new phenomena emerge, which are absent for isolated H2O 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 (H2O)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 H2O+ ions. As electrons of low energy (∼0.7-20 eV) have recently been found to efficiently break-up DNA constituents, the observed decay channel might contribute as a source of electrons that can cause radiation damage in biological matter.