Pump–probe experiments with attosecond resolution are the key to understanding electronic dynamics in quantum systems. Isolated attosecond soft X-ray pulses produced with a Free-Electron Laser (FEL) have pulse energies up to hundreds of microjoules [1]. The intense pulse energies of attosecond soft X-ray FEL pulses are sufficient for nonlinear X-ray spectroscopy and enable attosecond pump-probe experiments. In this talk, I will first review the technical developments of time-resolved experiments on X-ray Free-Electron Laser (XFEL) facilities. I will present the generation and control of sub-femtosecond pulse pairs from a two-color X-ray FEL [2]. By measuring the delay between the two pulses using an angular streaking diagnostic, we characterize the group velocity of the X-ray FEL and demonstrate the control of the pulse delay down to 270 as. We demonstrate the applicability of this technique to a pump–probe measurement in core-ionized para-aminophenol. These results reveal the ability to perform pump–probe experiments with sub-femtosecond resolution and atomic site specificity.
The highly flexible design of the Athos soft X-ray beamline at SwissFEL enables the generation of isolated attosecond pulses [3] and holds the potential for advanced X-ray attosecond spectroscopies. We are currently working on designing an angular streaking instrument at the Maloja endstation to diagnose isolated attosecond X-ray pulses produced at SwissFEL.
REFERENCES
[1] Duris, J., et al. Nature Photonics 14.1 (2020): 30-36.
[2] Guo, Z., et al. Nature Photonics (2024): 1-7.
[3] Prat, E., et al. APL Photonics 8.11 (2023).