Record Details

Structural modification of solids by ultra-short X-ray laser pulses
EuXFEL staff
Author group:
X-Ray Optics
Scientific area:
The motivation behind this Ph.D. project is to determine the structural modification of solids by ultra-short X-ray laser pulses. This Ph.D. project focuses on determining the amorphous carbon (a-C) as a potential coating on the mirrors of the soft X-ray beamline of the European X-ray Free Electron Laser (XFEL) in Hamburg, in particular. Furthermore, chemical vapor deposition (CVD) diamond used in the monochromators for X-ray beamlines of European XFELneeds to be examined. Among high Z materials Nickel (Ni), MoB4C (multi-layer), are studied at 269 eV photon energy. The focus was on testing the behavior of a-C coated mirrors and the CVD diamond monochromators which are the main subject in the performed experiments. XFEL deliver high peak brilliance, high power, femtosecond focused laser pulses. Optical elements in these facilities are of crucial importance as they should distribute the beam with high quality and survive the intense conditions. Hence, understanding the interplay between the X-ray FEL pulses with coatings on the mirrors as well as single crystal monochromators is important. By means of this project it becomes evident that from the fundamental aspect, different mechanisms are involved in the damage process at different time scales. In the early femtosecond (fs) time zone, the photo-ionization is the main mechanism governing the damage process. During this time the material density changes. The system tends to reach its energetically stable potential state (a-C turns into graphite). In the picosecond (ps) time scale, secondary processes initiate. Among those, one can mention Auger, impact ionization, tunnel ionization, carrier diffusion followed by free carriers interaction with the lattice e.g. electron-phonon coupling, etc. The heat diffusion process starts to take place after some 100 ps, which continues until the system returns to room temperature after some μs (7μs). The analysis of the damage process can be divided into three main phases; based on the different time zone named above. The combination of heat diffusion and secondary processes cause a nonlinear increase in the size of the damage spots on the logarithmic axis dependingon the pulse energy. The photo-ionization (non-thermal) damage threshold is determined from experiments performed at different FEL facilities on different photon energies.From heat diffusion simulation via COMSOL (software package based on advanced numerical methods), one can extract the melting energy threshold for each material at different photon energies. To gain a deeper knowledge on the damage process within the scope of this project, several investigations such as Atomic Force Microscopy (AFM), Raman spectroscopy, photoemission spectroscopy, and theoretical simulation via Hybrid XTANT code were conducted based on the subjected samples.
Schenefeld, Germany, European X-Ray Free-Electron Laser Facility GmbH, 2017
Related external records:
urn: nbn:de:gbv:18-88429
Degree type:
Ph.D. Thesis
Starting date:
Ending date:
Year granted:
University name:
University of Hamburg


 Record created 2019-10-01, last modified 2019-10-01

THESIS-2017-005_Dastjani_Farahani - Download fulltextPDF
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