High-energy-resolution off-resonant spectroscopy with self-seeded x-ray free-electron laser pulses.

This paper presents the implementation of high-energy-resolution off-resonant spectroscopy (HEROS) measurements using self-seeded x-ray free-electron laser (XFEL) pulses. This study systematically investigated XFEL conditions, including photon energy and accumulated shot numbers, to optimize the measurement efficiency for copper foil samples near the K-edge. The x-ray absorption spectra reconstructed using HEROS were compared with those derived from fluorescence-yield measurements. The HEROS-based spectra exhibited consistent line shapes independent of the sample thickness. The potential application of HEROS to high-temperature copper was also explored. HEROS offers distinct advantages including scan-free measurement of x-ray absorption spectra with reduced core-hole lifetime broadening and self-absorption effects. Using self-seeded XFEL pulses, HEROS facilitates single-shot-based pump-probe measurements to investigate the ultrafast dynamics in various materials and diverse conditions.

Resonant X-ray emission spectroscopy using self-seeded hard X-ray pulses at PAL-XFEL.

Self-seeded hard X-ray pulses at PAL-XFEL were used to commission a resonant X-ray emission spectroscopy experiment with a von Hamos spectrometer. The self-seeded beam, generated through forward Bragg diffraction of the [202] peak in a 100 µm-thick diamond crystal, exhibited an average bandwidth of 0.54 eV at 11.223 keV. A coordinated scanning scheme of electron bunch energy, diamond crystal angle and silicon monochromator allowed us to map the Ir Lß2 X-ray emission lines of IrO2 powder across the Ir L3-absorption edge, from 11.212 to 11.242 keV with an energy step of 0.3 eV. This work provides a reference for hard X-ray emission spectroscopy experiments utilizing self-seeded pulses with a narrow bandwidth, eventually applicable for pump-probe studies in solid-state and diluted systems.

Investigation of Nonequilibrium Electronic Dynamics of Warm Dense Copper with Femtosecond X-Ray Absorption Spectroscopy.

Ultrafast optical excitation of matter leads to highly excited states that are far from equilibrium. In this study, femtosecond x-ray absorption spectroscopy was used to visualize the ultrafast dynamics in photoexcited warm dense Cu. The rich dynamical features related to d vacancies are observed on femtosecond timescales. Despite the success in explaining x-ray absorption data in the picosecond regime, the new femtosecond data are poorly understood through the traditional two-temperature model based on the fast thermalization concept and the static electronic structure for high-temperature metals. An improved understanding can be achieved by including the recombination dynamics of nonthermal electrons and changes in the screening of the excited d block. The population balance between the 4sp and 3d bands is mainly determined by the recombination rate of nonthermal electrons, and the underpopulated 3d block is initially strongly downshifted and recovered in several hundreds of femtoseconds.

Femtosecond soft X-ray absorption spectroscopy of warm dense matter at the PAL-XFEL.

Free-electron laser pulse-based X-ray absorption spectroscopy measurements on warm dense copper are presented. The incident X-ray pulse energies were measured with a detector assembly consisting of a photocathode membrane and microchannel plates, and the transmitted energies were measured simultaneously with a photodiode detector. The precision of the absorption measurements was evaluated. For a warm dense copper foil irradiated by an intense femtosecond laser pulse, the enhanced X-ray absorption below the L3-edge, followed by the rapid evolution of highly excited Fermi liquid within a picosecond, were successfully measured. This result demonstrates a unique capability to study femtosecond non-equilibrium electron-hole dynamics in extreme states of matter.

Controlled reproducible alignment of cone targets and mitigation of preplasma in high intensity laser interactions.

The use of cone targets in high intensity laser-plasma experiments has been of recent interest because of their potential use in integrated fast ignition experiments. Simpler experiments provide a good avenue for understanding the underlying physics, however precise control of the alignment along with good pointing accuracy is of crucial importance. While on big laser facilities target alignment is done precisely with several microscopes, it is not always the case on smaller facilities. This can have a detrimental effect on the quality of the results. We have developed and characterized a method for accurate alignment of intense laser pulses into a cone target. This, along with optimal positioning of the focus compared to the tip, efficiently uses the shape of the target to microfocus the laser light and concentrates the hot electrons in the tip, and can mitigate preplasma issues.