Saturable absorption is a phenomenon readily seen in the optical and infrared wavelengths. It has never been observed in core-electron transitions owing to the short lifetime of the excited states involved and the high intensities of the soft X-rays needed. We report saturable absorption of an L-shell transition in aluminium using record intensities over 1016 W cm−2 at a photon energy of 92 eV. From a consideration of the relevant timescales, we infer that immediately after the X-rays have passed, the sample is in an exotic state where all of the aluminium atoms have an L-shell hole, and the valence band has approximately a 9 eV temperature, whereas the atoms are still on their crystallographic positions. Subsequently, Auger decay heats the material to the warm dense matter regime, at around 25 eV temperatures. The method is an ideal candidate to study homogeneous warm dense matter, highly relevant to planetary science, astrophysics and inertial confinement fusion.
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The authors want to acknowledge K. Budil of LLNL for assistance in support in funding, and acknowledge support for access to FLASH by DESY and the European Community under contract RII3-CT-2004-506008 (IA-SFS). The authors from Universität Rostock are supported by the Deutsche Forschungsgemeinschaft within SFB 652, B.N. by the EU Marie-Curie RTN ‘FLASH’, S.M.V. by EPSRC/STFC, W.M. by AWE, K.S. by the Slovak Grant Agency for Science (Grant No. 2/7196/27) and L.J., J.C., J.Ch. and V.H. by the Czech Ministry of Education (grants LC510, LC528 and LA08024) and Academy of Sciences of the Czech Republic (Z10100523, IAA400100701, and KAN 300100702). Technical assistance by A. Aquila, J. Meyer-Illse and E. M. Gullikson (LBNL) during the ALS beamtime is greatly appreciated. Operation of the Advanced Light Source was supported by the Director, Office of Science, Office of Basic Energy Sciences, of the US Department of Energy under Contract No. DE-AC03-76SF00098. We gratefully acknowledge financial support by the German Federal Ministry for Education and Research through project FSP 301-FLASH, and from the Ministry of Science and Higher Education of Poland through grant SPB No. DESY/68/2007. This work was in part carried out under the auspices of the US Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344, and in part supported by grants 08-ERI-002 and 08-LW-004.
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Nagler, B., Zastrau, U., Fäustlin, R. et al. Turning solid aluminium transparent by intense soft X-ray photoionization. Nature Phys 5, 693–696 (2009) doi:10.1038/nphys1341
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