Seminars Archive

Investigation of damage processes in carbon based solids induced by x-ray FEL pulses

Abstract
The progress of Free Electron Lasers (FELs) now makes available intense femtosecond light pulses in the x-ray domain. If FELs photon energy range is extending the energy per pulse is also increasing: several milli-Joule pulses have been demonstrated at LCLS and up-to 30 mJ will be delivered at the European XFEL. All the facilities rely on beamlinescomprising several components (mirrors, crystal monochromators…) to steer and manipulate the beam. It is then of a crucial importance to investigate and understand the interaction of solids with FEL pulsesto ensure that beamline components withstand the high photon flux. We will present results of experimentswhere carbon based materials (amorphous carbon (a-C) and single crystal diamond) have beenirradiated by sub-100 fs single shots at different photon energies ranging from 25 to 830 eV. The experiments were performed at different facilities: SCSS at Riken, FLASHin Hamburg and LCLS in Stanford.The damage fluencethresholdis determined ex-situ using optical microscopy. The results allow characterizing different aspects of the damage process: the photon energy dependence of the threshold, the effect of electron transport in the case of grazing incidence geometry and specific damage processfor typical optical elements like diffraction grating. From a more fundamental point of view, we characterized the irradiatedmaterialsby using different surface investigation methods: atomic force microscopy, micro-Raman and scanning photoelectron microscopy. The micro-Raman measurements evidence the ordering of the a-C leading to theformation of graphite nanocrystals. Photoelectron microscopy shows graphitization, i.e. conversion of the initial sp3bonds to sp2. In the case of diamond, the structure is shown to be directly transformed into graphite nanocrystals. These analyses allow identifying the damage process as a solid to solid phase transition. Finally, the possible non-thermal origin of the transition will be discussed.