Contrasting behavior of covalent and molecular carbon allotropes exposed to extreme ultraviolet and soft x-ray free-electron laser radiation

摘要

All carbon materials, e.g., amorphous carbon (a-C) coatings and C60 fullerenethin films, play an important role in short-wavelength free-electron laser(FEL) research motivated by FEL optics development and prospectivenanotechnology applications. Responses of a-C and C60 layers to the extremeultraviolet (SPring-8 Compact SASE Source in Japan) and soft x-ray(free-electron laser in Hamburg) free-electron laser radiation are investigatedby Raman spectroscopy, differential interference contrast, and atomic forcemicroscopy. A remarkable difference in the behavior of covalent (a-C) andmolecular (C60) carbonaceous solids is demonstrated under these irradiationconditions. Low thresholds for ablation of a fullerene crystal (estimated to bearound 0.15 eV/atom for C60 vs 0.9 eV/atom for a-C in terms of the absorbeddose) are caused by a low cohesive energy of fullerene crystals. An efficientmechanism of the removal of intact C60 molecules from the irradiated crystaldue to Coulomb repulsion of fullerene-cage cation radicals formed by theionizing radiation is revealed by a detailed modeling.