Understanding the dynamics of electron-phonon and phonon-phonon interactions is important to unravel the complex behavior of materials subject to ultrafast laser excitation. We report the results of studying these interactions in femtosecond laser-excited tungsten (W) using the technique of ultrafast electron diffuse scattering (UEDS). By tracking changes of diffuse scattering signal over time, we resolve the dynamics of phonon populations across the Brillouin zone in W. Our results shed light on both electron-phonon and phonon-phonon coupling dynamics in W [Mo et al. Science Advances, in press (2024)]. This paper outlines the fundamental principle behind the UEDS technique, provides a brief overview of the experimental setup, and presents selected results of time-resolved diffuse scattering patterns.
X-ray Free Electron Laser (XFEL) radiation may transform diamond into graphite. Two X-ray pulses were used; the first as pump to trigger the phase transition and the second as probe performing X-ray diffraction. The experiment was performed at the SACLA XFEL facility at the beamline 3 experimental hutch 5. The samples were polycrystalline diamond. The pump and probe photon energies were 7 and 10.5 keV, respectively, and the delay between the X-ray pulses was varied from 0 to 286 fs. To provide a range of energy densities, the X-ray focus was adjusted between 150 nm and 1 um. The (111), (220) and (311) diffraction peaks were observed. The intensity of each diffraction peak decreased with time indicating a disordering of the crystal lattice. From a Debye-Waller analysis, the root-mean-square (rms) atomic displacement perpendicular to particular lattice planes are calculated. At higher fluences, the rms atomic displacement perpendicular to the (111) planes is significantly larger than that perpendicular to the (220) or (311) planes. By accepting two successive XFEL pulses at a time delay of 33 ms, graphite (002) diffraction was observed beginning at a threshold dose of 1.7 eV/atom. The experimental results will be compared with calculations using a hybrid model based on tight-binding molecular dynamics.
We present the study of optical and spectral properties of radiation-induced stable point defects, known as color centers (CCs), in lithium fluoride (LiF) for the detection of 10 keV XFEL beam at Spring-8 Angstrom Compact free electron LAser (SACLA) in Japan. A thick LiF crystal was irradiated in four spots with 10 keV XFEL beam (pulse duration = 10 fs) with different number of accumulated shots. After irradiation the colored-LiF spots were characterized with an optical microscope in fluorescence mode and their photoluminescence intensity and spectra were analyzed.
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