Pokapanich, W.; Ottosson, N.; Svensson, S.; Öhrwall, G.; Winter, B.; Björneholm, O.: Bond Breaking, Electron Pushing, and Proton Pulling: Active and Passive Roles in the Interaction between Aqueous Ions and Water as Manifested in the O 1s Auger Decay. The Journal of Physical Chemistry B 116 (2012), p. 3-8
A core-ionized H2O molecule in liquid water primarily relaxes through normal Auger decay, leading to a two-hole final state in which both valence holes are localized on the same water molecule. Electronic coupling to the environment, however, allows for alternative decays resembling Intermolecular Coulombic Decay (ICD), producing final states with one of the holes delocalized on a neighboring water molecule. Here we present an experimental study of such minority processes, which adds to our understanding of dynamic interactions of electronically excited H2O molecules with their local surrounding in liquid water and aqueous solution. We show that the solvation of metal-halide salts considerably influences these minority decay channels from the water O 1s–1 state. By breaking water–water bonds, both the metal cations and halide anions are found to reduce the decay into water–water delocalized states, thus having a ″passive″ effect on the Auger spectrum. The halide anions also play an ″active″ role by opening a new ICD-like decay pathway into water-halide delocalized states. The importance of this contribution increases from F– to I–, which we suggest to be caused by a directional polarization of the halide anion toward the core-ionized H2O+ cation in the intermediate state of the Auger process. This increases the electronic overlap between the two centers and makes delocalized decays more probable. We furthermore show that F–, the smallest and most strongly hydrated of the halides, plays an additional role as proton puller during the core-hole lifetime, resulting in proton dynamics on the low femtosecond time scale. Our results represent a step forward toward a better understanding of how aqueous solutions, when exposed to soft X-rays, channel excess energy. This has implications for several aspects of physical and radiation chemistry, as well as biology.