Development of an experimental apparatus to observe ultrafast phenomena by tender X-ray absorption spectroscopy at PAL-XFEL

Understanding the ultrafast dynamics of molecules is of fundamental importance. Time-resolved X-ray absorption spectroscopy (TR-XAS) is a powerful spectroscopic technique for unveiling the time-dependent structural and electronic information of molecules that has been widely applied in various fields. Herein, the design and technical achievement of a newly developed experimental apparatus for TR-XAS measurements in the tender X-ray range with X-ray free-electron lasers (XFELs) at the Pohang Accelerator Laboratory XFEL (PAL-XFEL) are described. Femtosecond TR-XAS measurements were conducted at the Ru L 3-edge of well known photosensitizer tris(bipyridine)ruthenium(II) chloride ([Ru(bpy)3]2+) in water. The results indicate ultrafast photoinduced electron transfer from the Ru center to the ligand, which demonstrates that the newly designed setup is applicable for monitoring ultrafast reactions in the femtosecond domain.

Recent Progress of the PAL-XFEL after User Service Start

The X-ray Free Electron Laser of the Pohang Accelerator Laboratory (PAL-XFEL) was open to the public in 2017 and started user service. Even after the user service had started, improvement of beam quality was continuously attempted, and several meaningful results were achieved. In this paper, we present recent progress in the PAL-XFEL, including the present status of its operation. The increase in the FEL pulse energy is explained, and the generation of a self-seeding FEL is described. In addition, the generation of a two-color FEL is mentioned.

Single-Shot Coherent X-Ray Imaging Instrument at PAL-XFEL

We developed a single-shot coherent X-ray imaging instrument at the hard X-ray beamline of the Pohang Accelerator Laboratory X-ray Free Electron Laser (PAL-XFEL). This experimental platform was established to conduct a variety of XFEL experiments, including coherent diffraction imaging (CDI), X-ray photon correlation spectroscopy (XPCS), and coherent X-ray scattering (CXS). Based on the forward-scattering geometry, this instrument utilizes a fixed-target method for sample delivery. It is well optimized for single-shot-based experiments in which one expects to observe the ultrafast phenomena of nanoparticles at picosecond temporal and nanometer spatial resolutions. In this paper, we introduce a single-shot coherent X-ray imaging instrument and report pump–probe coherent diffraction imaging (PPCDI) of Ag nanoparticles as an example of its applications.

Генерация гармоник в экспериментах с лазерами на свободных электронах в рентгеновском диапазоне --- теоретический анализ

Analytical description of the generation of harmonics of the undulator radiation (UR) in free electron lasers (FELs) is given for various FEL experiments in the X-ray band. The expressions for the spectral line shape and intensity are written explicitly in terms of the generalized Bessel and Airy functions with account for the electron energy spread, beam sizes and emittances, spectral line split and non-periodic magnetic fields. The presented theory describes well the radiation spectral properties and harmonic intensities in all studied FEL experiments: SACLA, LCLS, PAL-XFEL, SwissFEL et al., in various conditions with the radiation in wide range of frequencies, covering hard X-rays.

Investigation of the damping wiggler effect and application on the PAL fourth-generation storage ring

An investigation of the damping wiggler effect to reduce the emittance in the Pohang Accelerator Laboratory (PAL), a fourth-generation storage ring (4GSR) that uses a multi-bend achromat, is presented. A 4GSR lattice which has reduced emittance and increased dynamic aperture to amplify the synergy with two existing light sources (PLS-II and PAL-XFEL) at PAL is described.