Rolf Mitzner, Jens Rehanek, Jan Kern, Sheraz Gul, Johan Hattne, Taketo Taguchi, Roberto Alonso-Mori, Rosalie Tran, Christian Weniger, Henning Schröder, Wilson Quevedo, Hartawan Laksmono, Raymond G. Sierra, Guangye Han, Benedikt Lassalle-Kaiser, Sergey Koroidov, Katharina Kubicek, Simon Schreck, Kristjan Kunnus, Maria Brzhezinskaya, Alexander Firsov, Michael P. Minitti, Joshua J. Turner, Stefan Möller, Nicholas K. Sauter, Michael J. Bogan, Dennis Nordlund, William F. Schlotter, Johannes Messinger, Andrew S. Borovik, Simone Techert, Frank M. F. de Groot, Alexander Föhlisch, Alexei Erko, Uwe Bergmann, Vittal K. Yachandra, Philippe Wernet, Junko Yano

• L-edge spectroscopy of 3d transition metals provides important electronic structure information and has been used in many fields. However, the use of this method for studying dilute aqueous systems, such as metalloenzymes, has not been prevalent because of severe radiation damage and the lack of suitable detection systems. Here we present spectra from a dilute Mn aqueous solution using a high-transmission zone-plate spectrometer at the Linac Coherent Light Source (LCLS). The spectrometer has been optimized for discriminating the Mn L-edge signal from the overwhelming 0 K-edge background that arises from water and protein itself, and the ultrashort LCLS X-ray pulses can outrun X-ray induced damage. We show that the deviations of the partial-fluorescence yield-detected spectra from the true absorption can be well modeled using the state-dependence of the fluorescence yield, and discuss implications for the application of our concept to biological samples.