Attosecond time–energy structure of X-ray free-electron laser pulses

• DOI: 10.1038/s41566-018-0107-6
• Publication Date: 2018-03-02T09:09:47Z
• Authors: N. Hartmann; G. Hartmann; R. Heider; M. S. Wagner; M. Ilchen; J. Buck; A. O. Lindahl; C. Benko; J. Grünert; J. Krzywinski; J. Liu; A. A. Lutman; A. Marinelli; T. Maxwell; A. A. Miahnahri; S. P. Moeller; M. Planas; J. Robinson; A. K. Kazansky; N. M. Kabachnik; J. Viefhaus; T. Feurer; R. Kienberger; R. N. Coffee; W. Helml
• Keywords: Attosecond; Streaking; Chirp

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.