The time–energy information of ultrashort X-ray free-electron laser pulses generated by the Linac Coherent Light Source is measured with attosecond resolution via angular streaking of neon 1s photoelectrons. The X-ray pulses promote electrons from the neon core level into an ionization continuum, where they are dressed with the electric field of a circularly polarized infrared laser. This induces characteristic modulations of the resulting photoelectron energy and angular distribution. From these modulations we recover the single-shot attosecond intensity structure and chirp of arbitrary X-ray pulses based on self-amplified spontaneous emission, which have eluded direct measurement so far. We characterize individual attosecond pulses, including their instantaneous frequency, and identify double pulses with well-defined delays and spectral properties, thus paving the way for X-ray pump/X-ray probe attosecond free-electron laser science.

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We would like to thank C. Bostedt for his extensive and skilful support during the beam time. This research was carried out at the Linac Coherent Light Source (LCLS) at the SLAC National Accelerator Laboratory. LCLS is an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science by Stanford University. W.H. acknowledges financial support from a Marie Curie International Outgoing Fellowship. W.H., R.H., M.S.W. and R.K. acknowledge financial support by the BaCaTeC program, the German Cluster of Excellence MAP and the European I3-Initiative 'LASERLAB-Europe IV'. R.K. acknowledges an ERC Consolidator Grant 'AEDMOS'. J.K., A.A.L, A.M., T.M., S.M., R.N.C. and J.R. acknowledge DOE support under contract DE-AC02-76SF00515. N.M.K. acknowledges financial support from the theory group in cooperation with the SQS work package of European XFEL. A.K.K. acknowledges financial support from the project FIS2016-76617-P of MINECO. T.F. acknowledges financial support from the National Center of Competence in Research, Molecular Ultrafast Science and Technology. M.I. acknowledges funding of the Volkswagen Foundation within a Peter Paul Ewald-Fellowship.

Author information

Author notes

    • N. Hartmann

    Present address: Coherent Inc., Santa Clara, CA, USA


  1. SLAC National Accelerator Laboratory, Linac Coherent Light Source, Menlo Park, CA, USA

    • N. Hartmann
    • , M. Ilchen
    • , A. O. Lindahl
    • , J. Krzywinski
    • , A. A. Lutman
    • , A. Marinelli
    • , T. Maxwell
    • , A. A. Miahnahri
    • , S. P. Moeller
    • , J. Robinson
    • , R. N. Coffee
    •  & W. Helml
  2. University of Bern, Institute of Applied Physics, Bern, Switzerland

    • N. Hartmann
    •  & T. Feurer
  3. Deutsches Elektronen-Synchrotron DESY, Hamburg, Germany

    • G. Hartmann
    • , J. Buck
    •  & J. Viefhaus
  4. Institut für Physik und CINSaT, Universität Kassel, Kassel, Germany

    • G. Hartmann
  5. Physik-Department E11, Technische Universität München, Garching, Germany

    • R. Heider
    • , M. S. Wagner
    • , R. Kienberger
    •  & W. Helml
  6. European XFEL GmbH, Schenefeld, Germany

    • M. Ilchen
    • , J. Buck
    • , J. Grünert
    • , J. Liu
    • , M. Planas
    •  & N. M. Kabachnik
  7. PULSE Institute, Stanford University and SLAC National Accelerator Laboratory, Menlo Park, CA, USA

    • M. Ilchen
    •  & R. N. Coffee
  8. University of Gothenburg, Department of Physics, Gothenburg, Sweden

    • A. O. Lindahl
  9. Qamcom Research & Technology AB, Göteborg, Sweden

    • A. O. Lindahl
  10. University of Colorado at Boulder, JILA, Boulder, CO, USA

    • C. Benko
  11. Departamento de Fisica de Materiales, University of the Basque Country UPV/EHU, San Sebastian/Donostia, Spain

    • A. K. Kazansky
  12. Donostia International Physics Center (DIPC), San Sebastian/Donostia, Spain

    • A. K. Kazansky
  13. IKERBASQUE, Basque Foundation for Science, Bilbao, Spain

    • A. K. Kazansky
  14. Skobeltsyn Institute of Nuclear Physics, Lomonosov Moscow State University, Moscow, Russia

    • N. M. Kabachnik
  15. Max-Planck-Institut für Quantenoptik, Garching, Germany

    • R. Kienberger
  16. Ludwig-Maximilians-Universität München, Garching, Germany

    • W. Helml


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N.H., M.I., A.O.L., R.N.C. and W.H. conceived and coordinated the experiment. N.H., R.H., M.S.W., C.B., J.L., A.A.M., S.P.M., J.R., R.N.C. and W.H. built the vacuum system and the optical set-up, and supervised them during the measurement. M.I, J.B., A.O.L., J.G., M.P. and J.V. were responsible for constructing, installing and operating the photoelectron detector. J.K., A.A.L., A.M. and T.M. carried out accelerator control and XFEL characterization. N.H., G.H., R.H., M.S.W., J.B., A.O.L., J.L., R.N.C. and W.H. performed data analysis. N.H., G.H., M.I., T.F., R.K., R.N.C. and W.H. interpreted data and oversaw manuscript production. A.K.K., N.M.K. and T.F. provided simulations for comparison with experimental data. N.H., G.H. and W.H. wrote the paper with extensive contributions from all authors.

Competing interests

The authors declare no competing interests.

Corresponding authors

Correspondence to N. Hartmann or W. Helml.

Supplementary information

  1. Supplementary Information

    Supplementary Methods; Supplementary Discussion; Supplementary Figures 1–8; Supplementary Table 1; Supplementary References 1–8.

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