Ultrafast spectroscopy: ejecting electrons from water.

T he hydrated electron, an electron in aqueous solution, has captured the attention of scientists since it was discovered in 1962 as a product of water radiolysis 1. Many properties of the hydrated electron are consistent with the 'cavity' model, in which it resides in a non-spherical solvent cavity with an average radius of ~2.4 Å and is stabilized by hydrogen-bonding interactions with, on average, six water molecules 2. This species plays a major part in radiation chemistry and biology: hydrated electrons can be formed by ionizing radiation in living cells, and their high reactivity leads to free-radical formation with significant potential for genetic damage. Moreover, hydrated electrons may be able to attack DNA directly, resulting in single-and double-strand breaks. On a more fundamental level, the hydrated electron represents the simplest quantum mechanical solute, motivating many studies of its spectroscopy, reactivity and relaxation dynamics. This research in aqueous solution has been complemented by parallel efforts in the gas phase on water cluster anions, (H 2 O) n − , in which an electron is bound to a known number (n) of water molecules 3. Experimental and theoretical work on these clusters has provided valuable insights into the nature of the hydrated electron, but has also raised the important conceptual issue of how the properties of finite clusters can be extrapolated to bulk aqueous solutions. Writing in Nature Chemistry, Bernd Abel and colleagues report 4 the first measurements of the energy needed to eject a hydrated electron from bulk liquid water into the vacuum without a change in solvent configuration, that is, its vertical binding energy (VBE). This experiment represents a breakthrough in our understanding of hydrated electrons on several fronts. First, the VBE itself, found to be 3.3 eV, is a key property that has been estimated but never directly determined before. It represents a sensitive probe of the electron–solvent interactions that govern many of the attributes of the hydrated electron and places it and its excited states on an absolute scale relative to the vacuum. Second, the experiment shows evidence for an alternative type of hydrated electron with a VBE of 1.6 eV, which is attributed to an electron bound at the surface of liquid water. Third, the results represent a 'missing link' between the hydrated electron and (H 2 O) n − clusters. VBEs of water cluster anions were first reported by Bowen et al 5. and extrapolated …