title: Origin of dark-channel X-ray fluorescence from transition-metal ions in water.
abstract: The nonradiative dark channels in the L-edge fluorescence spectra from transition-metal aqueous solution identify the ultrafast charge-transfer processes playing an important role in many biological and chemical systems. Yet, the exact origin of such spectral dips with respect to the X-ray transmission spectrum has remained unclear. In the present study we explore the nature of the underlying decay mechanism of 2p core-excited Co(2+) in water by probing the nonradiative Auger-type electron emission channel using photoelectron spectroscopy from a liquid microjet. Our measurements demonstrate unequivocally that metal-to-water charge transfer quenches fluorescence and will inevitably lead to a dip in the total-fluorescence-yield X-ray absorption spectrum. This is directly revealed from the resonant enhancement of valence signal intensity arising from the interference of two identical final states created by a direct and Auger-electron emission, respectively.
authors.affiliation: Helmholtz-Zentrum Berlin für Materialien und Energie, Albert-Einstein-Strasse 15, D-12489 Berlin, Germany.
authors.forename: Robert
authors.initials: R
authors.lastname: Seidel
authors.forename: Samira
authors.initials: S
authors.lastname: Ghadimi
authors.forename: Kathrin M
authors.initials: KM
authors.lastname: Lange
authors.forename: Sébastien
authors.initials: S
authors.lastname: Bonhommeau
authors.forename: Mikhail A
authors.initials: MA
authors.lastname: Soldatov
authors.forename: Ronny
authors.initials: R
authors.lastname: Golnak
authors.forename: Alexander
authors.initials: A
authors.lastname: Kothe
authors.forename: René
authors.initials: R
authors.lastname: Könnecke
authors.forename: Alexander
authors.initials: A
authors.lastname: Soldatov
authors.forename: Stephan
authors.initials: S
authors.lastname: Thürmer
authors.forename: Bernd
authors.initials: B
authors.lastname: Winter
authors.forename: Emad F
authors.initials: EF
authors.lastname: Aziz