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Ultra-efficient ionization of heavy atoms by intense X-ray free-electron laser pulses

Abstract

X-ray free-electron lasers provide unique opportunities for exploring ultrafast dynamics and for imaging the structures of complex systems. Understanding the response of individual atoms to intense X-rays is essential for most free-electron laser applications. First experiments have shown that, for light atoms, the dominant interaction mechanism is ionization by sequential electron ejection, where the highest charge state produced is defined by the last ionic state that can be ionized with one photon. Here, we report an unprecedentedly high degree of ionization of xenon atoms by 1.5 keV free-electron laser pulses to charge states with ionization energies far exceeding the photon energy. Comparing ion charge-state distributions and fluorescence spectra with state-of-the-art calculations, we find that these surprisingly high charge states are created via excitation of transient resonances in highly charged ions, and predict resonance enhanced absorption to be a general phenomenon in the interaction of intense X-rays with systems containing high-Z constituents.

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Figure 1: Comparison of experimental and simulated xenon charge state yields.
Figure 2: Ground state binding energies for M-, N- and O-shell electrons in xenon ions for charge states between Xe14+ and Xe38+ together with schematic ionization pathways.
Figure 3: Calculated photoabsorption cross-sections (in megabarn, Mb) in Xeq+ (q > 21) around 1.5 keV.
Figure 5: Xenon ion yield for selected charge states Xeq+ as a function of X-ray fluence.
Figure 4: Xenon fluorescence at an incident photon energy of 1.5 keV.

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Acknowledgements

Experiments were carried out at the LCLS, a national user facility operated by Stanford University on behalf of the US Department of Energy (DOE), Office of Basic Energy Sciences. The authors acknowledge support from the Max Planck Society for funding the development and operation of the CAMP instrument within the ASG at CFEL. N.B. and A.R. acknowledge the US DOE, Office of Science, Basic Energy Science, Chemical, Geosciences, and Biological Divisions, K.U. acknowledges NEXT, Japan Society for the Promotion of Science (JSPS) and Institute of Multidisciplinary Research for Advanced Materials (IMRAM), and M.A., T.G., T.M., D.Ru. and S.S. acknowledge funding through Federal Ministry of Education and Research (BMBF) project 05KS4KTC1, Deutsche Forschungsgemeinschaft (DFG) project BO 3169/2-2, and Helmholtz-Gemeinschaft Deutscher Forschungszentren (HGF) Virtuelles Institut VHVI- 302. The authors thank T. Erke and J-H. Meyer for technical help during the setup of CAMP and thank the staff of the LCLS, in particular P. Anthony, J-C. Castagna, C. Rago, C. O'Grady, M. Weaver, A. Perazzo, R. Rodriguez, C. Brown and D. Cox for their support in preparing for and carrying out these experiments. The authors also thank U. Saalmann for helpful discussions and preliminary calculations in the initial phase of the data analysis and for careful reading of the manuscript.

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J.U., A.R. and D.Ro. conceived and coordinated the experiment, which was designed by S.W.E., R.H., K-U.K., R.M., I.S., C.-D.S., J.U., A.R. and D.Ro. The CAMP instrument was conceived by J.U. and set up and commissioned by B.R., L.M.F., S.W.E., B.E., R.H., M.A., A.A., C.B., N.C., F.F., T.G., L.G., H.G., H.H., A.H., C.K., F.K., K-U.K., M.Ma., B.N., G.P., D.Ru., C.S., S.S., J.S., J.U., A.R. and D.Ro. The pnCCD detectors were developed and operated by S.W.E., R.H., R.A., H.Go., G.H., S.H., P.H., N.K., D.M., D.P., C.R., G.S., F.S., H.S., L.S. and G.W. The beamline was set up and operated by C.B., J.D.B. and M.Me. The experiment was carried out by B.R., L.M.F., S.W.E., B.E., R.H., M.A., A.A., N.B., C.B., T.G., L.J., N.K., K-U.K., T.M., K.N., G.P., C.R., D.Ru., C.S., S.S., M.S., K.U., J.U., A.R. and D.Ro. The data were analysed by B.R., L.M.F., B.E., A.H., A.R. and D.Ro. Theoretical calculations were performed by S-K.S. and R.S., and B.R., S-K.S., J.U., A.R. and D.Ro wrote the manuscript with discussion and improvements from all authors.

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Correspondence to Daniel Rolles.

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Rudek, B., Son, SK., Foucar, L. et al. Ultra-efficient ionization of heavy atoms by intense X-ray free-electron laser pulses. Nature Photon 6, 858–865 (2012). https://doi.org/10.1038/nphoton.2012.261

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