# Data Acquisition System Based on the Bluesky Suite in the Shanghai Synchrotron Radiation Facility
## 2 Materials and Methods  
### Sub-Assemblies  
X-ray absorption spectroscopy (XAS) is a technique that probes the local atomic structure and chemical environment of elements in a material by measuring the attenuation of X-rays as they pass through a sample [18; 19; 20]. The X-ray absorption spectrum obtained from XAS is characterized by a series of peaks and edges that correspond to the absorption of X-rays by the different electron shells of the absorbing atoms. XAS comprises two regions: X-ray absorption near edge structure (XANES/NEXAFS), which provides information about the unoccupied electronic states of the absorbing atoms, and extended fine structure (EXAFS), which provides information about the local atomic environment of the absorbing atoms. As a scientific experimental method, XAS plays an important role in material, physics, chemistry, and life sciences research [21; 22; 23; 24; 25]. XAS can be collected in multiple ways, such as the total electron yield (TEY) method, the total fluorescence yield (TFY) method, the constant final state (CFS) method, and the constant initial state (CIS) method [26]. The TEY method with the sample current mode is widely used because it can be obtained usinga relatively simple experimental setup. However, because the escape depth of an electron in solids is from a few to a few-to-ten angstrom, the TEY method is employed to obtain sample surface information [27; 28]. The TFY method measures the fluorescent X-ray that can arise from a deeper region of the sample. The BL07U beamline adopts the TEY method and the TFY method for XAS. The BL07U beamline is a new extreme ultraviolet and soft X-ray beamline in the SSRF Phase-II Project. BL07U is the combination of XAS, X-ray magnetic circular dichroism (XMCD), X-ray magnetic linear dichroism (XMLD), Nano resolved-Angle resolved photoemission spectroscopy (Nano-ARPES) and Spin resolved-Angle resolved photoemission spectroscopy (Spin-ARPES) beamline. The beamline is equipped with a pair of elliptical polarized undulators (EPUs) and a variable-included-angle plane-grating monochromator, and delivers circularly polarized or linear-polarized X-rays in the energy range from 50 to 2000 eV. The beamline has two branches: one dedicated to Nano-ARPES, which has a minimum spot size of only 200 nm, and another branch dedicated to Spin-ARPES, vector magnetic field and high magnetic field endstations. The energy-resolving power of BL07U is over 9000 at 867 eV and 15,000 at 91 eV. The photo flux of BL07U is over 2.5 \(\times\) 10\({}^{11}\) photons/s/0.01%BW at 867 eV and 2.6 \(\times\) 10\({}^{9}\) photons/s/0.01%BW at 91 eV. The spot size is 150 \(\times\) 50 \(\upmu\)m\({}^{2}\) at 91 eV. The TEY and TFY experiments can be carried out simultaneously at the BL07U beamline. Figure 1 shows the hardware of the BL07U XAS measurement system. It consists of 5 channels for measurement. The incident X-ray photon flux I\({}_{0}\) can be measured in two channels. One channel detects I\({}_{0}\) by a toroidal mirror (TM) with a gilded surface in the X-ray light path. Another channel detects I\({}_{0}\) by a clean gold mesh in the X-ray light path. The sample signal can be obtained through three other channels. The standard sample current is connected to the input of the current amplifier by using a shield cable. The standard sample has a known high-purity composition; hence, it is commonly used to calibrate and test the performance of instruments at the beamline. It can also be used to infer the composition of the sample to be analyzed by comparing the spectra, as well as to reduce the error on the beam energy calibration if it is acquired simultaneously. The TFY method measures the fluorescent X-ray that can arise from a deeper region of the sample. For the TEY experiment, the sample current is also connected to the input of the current amplifier via a shield cable. For the TFY experiment, the fluorescence signal of the sample is detected by a photodiode (PD) that was placed in front of the sample and outside the X-ray light path.