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

TL;DR

This study compares how covalent amorphous carbon and molecular C60 fullerene respond to extreme ultraviolet and soft x-ray laser radiation, revealing distinct ablation thresholds and mechanisms, with implications for nanotechnology and FEL applications.

Contribution

It demonstrates the contrasting behaviors of covalent and molecular carbon allotropes under FEL radiation, highlighting different ablation thresholds and removal mechanisms.

Findings

C60 has a lower ablation threshold (~0.15 eV/atom) than amorphous carbon (~0.9 eV/atom).

Coulomb repulsion causes removal of intact C60 molecules from irradiated crystals.

Distinct responses of covalent and molecular carbons to FEL radiation are experimentally demonstrated.

Abstract

All carbon materials, e.g., amorphous carbon (a-C) coatings and C60 fullerene thin films, play an important role in short-wavelength free-electron laser (FEL) research motivated by FEL optics development and prospective nanotechnology applications. Responses of a-C and C60 layers to the extreme ultraviolet (SPring-8 Compact SASE Source in Japan) and soft x-ray (free-electron laser in Hamburg) free-electron laser radiation are investigated by Raman spectroscopy, differential interference contrast, and atomic force microscopy. A remarkable difference in the behavior of covalent (a-C) and molecular (C60) carbonaceous solids is demonstrated under these irradiation conditions. Low thresholds for ablation of a fullerene crystal (estimated to be around 0.15 eV/atom for C60 vs 0.9 eV/atom for a-C in terms of the absorbed dose) are caused by a low cohesive energy of fullerene crystals. An efficient mechanism of the removal of intact C60 molecules from the irradiated crystal due to Coulomb repulsion of fullerene-cage cation radicals formed by the ionizing radiation is revealed by a detailed modeling.