Quantum mechanics

Free-electron laser

Materials science

Streaking

Energy (signal processing)

Atomic physics

Argon

Laser

Physics

Ionization

Neon

Optics

Free electron model

Electron

X-ray

Femtosecond

Biophotonics

Optoelectronics

Attosecond

Chirp

Ultrashort pulse

Nature Photonics

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. Attosecond time–energy characterization of pulses generated by the Linac Coherent Light Source (LCLS) X-ray free-electron laser is enabled by angular streaking measurements.

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