Negative-Ion/Positive-Ion Coincidence Yields of Core-Excited Water

The analysis of negative-ion/positive-ion (NIPICO) and negative-ion/positive-ion/positive-ion coincidence (NIPIPICO) events provides new information on pathways leading to negative ion production in water.
C. Stråhlman et al.; J. Phys. Chem. (2016)

Molecules composed of light elements usually break into parts after the absorption of an X-ray photon. This dissociation mostly produces positive and neutral fragments owing to the positive charge of the molecular ion after normal and resonant Auger decay, which are the predominant decay channels of core-ionized and core-excited states, respectively. However, negative ions have also been observed at the core edges of several small molecules such as CO2, H2O and SF6.
            We report yields of mass-resolved negative ions and positive ions measured in coincidence after core excitation of water molecules. Our results complement earlier measurements of noncoincident negative-ion yieldsand coincident positive-ion yieldsin this energy region. O- observedin coincidence with H+ ions was the strongest observed NIPI(PI)CO channel. H- wasobserved in coincidence with H+ , O+ , and O2+ . NIPICO channels are observed both below and above the O 1s ionization threshold. Fragmentation following (resonant) Auger decay of core−hole states is the main pathway to the production of negative ions, but we have also observed evidence that the radiative decay of core−hole states contributes to the negative ion production above the O 1s IP. An increase in the H-/O+ yield around 556 eV is attributed to nonradiative decay of doubly excited states. The data show that negative-ion/positive-ion coincidence measurements can be readily utilized to disentangle unknown decay and dissociation processes involving negative-ion production. Negative ion production, which is typically orders of magnitude less probable than that of positive ions, provides a very sensitive probe for fragmentation studies



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Negative-Ion/Positive-Ion Coincidence Yields of Core-Excited Water
Christian Stråhlman,Antti Kivimäki,Robert Richter,and Rami Sankari
J. Phys. Chem. A 2016, 120, 6389


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