Radiation chemistry: Radical water.

I onizing radiation is known to be harmful to living organisms because it creates reactive radicals and charged species. In particular, highly energetic X-rays that ionize by removing one of the innermost (core) electrons of an atom lead to a cascade of events that must be better characterized if we are to fully understand their toxic effects 1. Writing in Nature Chemistry, Petr Slavíček, Bernd Winter and colleagues 2 have now described previously unknown steps within this cascade that involve proton transfer. Core electrons are considered chemically inert, but they have the important role of screening the electric field of the nucleus — each core electron effectively cancelling the field of one proton. Now consider the absorption of an X-ray photon with enough energy to eject one of the core electrons of an atom, effectively removing one screening charge. If the atom is oxygen, its valence electrons suddenly feel an attraction similar in strength to that generated by a fluorine nucleus, which has one additional proton. If the oxygen is in a water molecule in solution, the core ionization effectively transforms the molecule to H 2 F + hydrogen-bonded, for example, to water. The H 2 F + is very acidic, and this strongly favours the transfer of a proton to water to make HF and H 3 O +. But the atom masquerading as a fluorine atom is of course an oxygen atom that is highly excited because of the empty position in its lowest electron level. The lifetime of this excited species is very short, ~4 fs, because the hole can be easily filled by a valence electron. This process releases energy that can be removed either by emission of an X-ray photon or, more commonly, through ejection of another valence electron (this is called Auger decay). The electron can be ejected either from the same molecule or, if there is some orbital overlap, from a different one; both cases lead to a doubly positive final state. Slavíček, Winter and colleagues 2 are now looking to understand the effects of the proton transfer reaction on the electron ejection processes and species produced. And the tool that they use is Auger spectroscopy, which measures the kinetic energy of the ejected electron. What is unique in hydrogen-bonded systems is the ease with which the protons delocalize between strongly interacting water molecules on core ionization (see Fig. 1); this is due …