# Single-Shot Coherent X-Ray Imaging Instrument at PAL-XFEL
## 3 Results  
We conducted a single-shot PPCDI experiment to observe melting process of Ag nanosphere. The experiment was performed at the NCI endstation in the hard X-ray beamline of PAL-XFEL. It was designed to show a typical application of the developed instrument. A femtosecond IR laser with a 100-fs pulse duration was used as a pump source. The pump laser with 180 \(\upmu\)m \(\times\) 200 \(\upmu\)m size (FWHM) was illuminated on the sample plane. The laser fluence was -420 mJ/cm\({}^{2}\), which can increase sample temperature up to 1290 K [31]. As the melting temperature of Ag is 1235 K, this fluence is enough to cause thermal melting of Ag nanoparticles. The X-ray energy was fixed to 5 keV, which corresponds to 0.248 nm in wavelength. A KB mirror was used to deliver a focused beam with 7 \(\upmu\)m \(\times\) 9 \(\upmu\)m size (FWHM). Highly monodisperse Ag nanospheres with 100 nm diameter (nanoComposix, San Diego, CA, USA) were prepared for the sample. Before dispersing nanospheres on the Si\({}_{3}\)N\({}_{4}\) membrane, the membrane surface was charged to increase the wettability by using a plasma cleaner. Ag nanospheres were ultra-sonicated to avoid the aggregation. Additionally, a spin coater was used to promote uniform dispersion on the membrane [22]. Fresh specimens were provided to each X-ray pulse by using high-speed piezo stages. Diffraction patterns were collected through the MPCCD detector. The detector was placed 1.5 m downstream of the sample. Diffraction patterns were classified according to time delay, hit or nonhit status, and single or multiple particles. Figure 6a,c,d show three representative diffraction patterns from single Ag nanoparticles acquired at different time delays: 0 ps, 16 ps, and 45 ps. For phase retrieval, a generalized proximal smoothing (GPS) algorithm was used [32]. The reconstructed images are shown in the insets of Figure 6a,c. A diffraction pattern, which is similar to the airy rings, reflects the shape and the size of the specimen. In the far field, the amplitude of the diffraction pattern is the Fourier transform of the electron density  
Figure 5: Timing synchronization of two light sources with femtosecond precision using a Ce:YAG. The intensity of the transmitted IR laser is kept constant at negative delays. The intensity is reduced when X-ray pulses arrive first. distribution of the sample. The samples to be measured are Ag nanospheres of uniform density, the diffraction amplitude \(\rm{F(s)}\) is as follows. \[\rm{F(s)}=\frac{4}{3}\pi R^{3}\Phi(2\pi sR), \tag{1}\]  
\[\rm{where,\ \Phi(u)=3\frac{(sin\,u-u\,cos\,u)}{u^{3}}}.\]  
Here, R is the radius of the sphere, and Equation (1) is the Fourier transform of a uniform sphere. The intensity of the measured diffraction pattern can be estimated from the square of Equation (1). Figure 6a shows the diffraction pattern measured on an intact Ag nanosphere. The blue dashed circle in Figure 6a represents the momentum transfer of the zeroth speckle, \(\rm{q_{zeroth}}\), and its radius is 0.08 nm\({}^{-1}\). The diameter of the actually measured Ag nanosphere can be estimated by Equation (1), and its value is ~112 nm. In addition, the radial intensity sum of the diffraction pattern in Figure 6a is fitted to the radial intensity sum of the  
Figure 6: Diffraction patterns measured over a series of time delays showing the Ag nanoparticle melting process. Ag nanoparticles with a nominal diameter of 100 nm were melted by the IR laser (\(\lambda\) = 800 nm, ~420 ml/cm\({}^{2}\) fluence). The intact Ag nanosphere reconstructed from diffraction pattern measured at 0 ps time delay is shown in the inset of **(a)**. The blue dashed circle of the diffraction pattern in **(a)** represents the size of the zeroth speckle. The \(\rm{q_{zeroth}}\) corresponding to the radius of this circle is 0.08 nm\({}^{-1}\).