Seidel, R.; Ghadimi, S.; Lange, K.M.; Bonhommeau, S.; Soldatov, M.A.; Golnak, R.; Kothe, A.; Könnecke, R.; Soldatov, A.; Thürmer, S.; Winter, B.; Aziz, E.F.: Origin of dark-channel X-ray fluorescence from transition-metal ions in water. Journal of the American Chemical Society 134 (2012), p. 1600-1605

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 Co2+ 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.