## 4 Recent Progress in Beamlines
### Time-Resolved X-ray Absorption Spectroscopy with Tender X-ray FEL  
Time-resolved X-ray absorption spectroscopy (TR-XAS) is one of the major experimental tools used in the chemical science program at PAL-XFEL. X-ray absorption spectroscopy is based on the electron transition between the core-level orbital and empty valence orbital. Because X-ray-induced electron transition starts from excitation at the core-level orbital,  
Figure 6: (**a**) Summary of optical laser wavelengths and their maximum energies available at the FXS endstation. Blue and pink bars are the laser energies confirmed using the 800 nm fundamental and its harmonics, and the OPA outputs, respectively. The pink-shadowed region denotes the achievable range of wavelengths and energies via OPA. The gray dashed rectangular region will be available upon introducing the plasma source and optical rectification. In general, for an insulating magnetic material, the photoexcitation in the range from UV to NIR leads to electronic excitation, while mid-IR and far-IR ranges drive phononic and magnonic excitations, respectively. (**b**) A photograph of the four-circle diffractometer with optical components placed near the sample on the goniometer at the FXS endstation. the X-ray absorption spectrum has atomic selectivity. The X-ray absorption spectrum is also affected by the energy level of the empty valence orbitals, which can be directly related to the structural and electrical environment around specific atoms. Therefore, it is possible to understand the electronic structure, spin state, and charge distribution, as well as the atomic structure, from the X-ray absorption spectrum. Applying pump-probe experiments, the TR-XAS can provide information about the structural and electronic changes during chemical reactions. Investigation of the photoinduced processes within sub-picoseconds (i.e., the natural timescales of elementary chemical reactions) by X-ray is now possible with femtosecond X-rays from an XFEL. Hard X-ray TR-XAS at the PAL-XFEL is possible at both FXL and CXI endstations [20, 46]. The difference between the two endstations is the focusing optics: compound refractive lens (CRL) for the FXL endstation and Kirkpatrick-Baez (K-B) mirror for the CXI endstation (Figure 7). For optimization of the performance of TR-XAS in a wide range of X-ray energy, K-B mirror based X-ray focus is an optimum choice. Therefore, we installed a time-resolved X-ray absorption measuring system at the CXI endstation [47]. For TR-XAS measurement, a monochromatized FEL using a double-crystal monochromator (DCM) is mandatory to increase the energy resolution. While passing through the DCM, the XFEL path length shows energy dependence. For example, at higher Bragg angles (low X-ray energy such as 2.8 keV, Ru L\({}_{3}\)-edge), a 10 eV energy change causes a 1 ps difference in arrival time of the XFEL to the sample. In experiments at the PAL-XFEL using DCM, the different arrival times of the FEL at different energy levels can be corrected by considering the geometric beam path difference inside the DCM. Herein, we introduce one representative result. The TR-XAS setup is shown in Figure 7. The chamber can provide a vacuum environment (\(\sim\)1 mbar) or 1 atm He environment to minimize the loss of X-ray flux. The liquid-phase sample is circulated through a 100 or 150 \(\upmu\)m thickness round nozzle using a magnetic gear pump. A focused 20 \(\times\) 20 \(\upmu\)m\({}^{2}\) (FWHM) X-ray beam with 60 Hz was used, and a 400 nm optical laser was delivered on the sample at 30 Hz with a pulse energy of 24 \(\upmu\)J. The optical laser [33] is focused to about 100-200 \(\upmu\)m at the sample position. To minimize the group velocity mismatch between the X-ray and the laser, they have collinear geometry. The avalanche photodiode (APD) monitors the total fluorescence yield from the sample. APD is shielded by 1.6 \(\upmu\)m Al foil to block the scattered pump laser.