Capturing Nonlinear Electron Dynamics with Fully Characterised Attosecond X-ray Pulses

TL;DR

This paper demonstrates the generation and full characterization of ultrashort, high-power attosecond X-ray pulses at ~1 keV, enabling detailed study of nonlinear electron dynamics in matter.

Contribution

It reports the first full pulse-to-pulse characterization of intense attosecond X-ray pulses from FELs using angular streaking, and applies this to observe nonlinear electron interactions in neon.

Findings

Achieved 600 attosecond pulse durations with 200 GW peak power.

Observed single- and double-core-hole generation in neon atoms.

Revealed intensity-dependent probabilities for double-core vacancy formation.

Abstract

Attosecond X-ray pulses are the key to studying electron dynamics at their natural timescale in specifically targeted electronic states. They promise to build the conceptual bridge between physical and chemical photo-reaction processes. Free-electron lasers (FELs) have demonstrated their capability of generating intense attosecond X-ray pulses. The use of SASE-based FELs for time-resolving experiments and investigations of nonlinear X-ray absorption mechanisms, however, necessitates their full pulse-to-pulse characterisation which remains a cutting-edge challenge. We have characterised X-ray pulses with durations of down to 600 attoseconds and peak powers up to 200 GW at ~1 keV photon energy via angular streaking at the Small Quantum Systems instrument of the European XFEL in Germany. As a direct application, we present results of nonlinear X-ray--matter interaction via time-resolved electron spectroscopy on a transient system, observing single- and double-core-hole generation in neon atoms. Using the derived temporal information about each single X-ray pulse, we reveal an otherwise hidden peak-intensity dependence of the probability for formation of double-core vacancies in neon after primary K-shell ionisation. Our results advance the field of attosecond science with highly intense and fully characterised X-ray pulses to the state-specific investigation of electronic motion in non-stationary media.