Amorphous to crystalline phase transition in carbon induced by intense femtosecond x-ray free-electron laser pulses

Abstract

We present the results of an experiment where amorphous carbon undergoes a phase transition induced by femtosecond 830 eV x-ray free-electron laser pulses. The phase transition threshold fluence is found to be 282 \ifmmode\pm\else\textpm\fi{} 11 mJ/cm${}^{2}$. Atomic force microscopy, photoelectron microscopy, and micro-Raman spectroscopy give experimental evidence for the phase transition in terms of a volume expansion, graphitization, and change of local order of the irradiated sample area. The interaction is modeled by an accurate time-dependent treatment of the ionization dynamics coupled to a two-temperature model. At the phase transition fluence threshold the free-electron density ${N}_{e}$ is found to be at maximum 9 \ifmmode\times\else\texttimes\fi{} 10${}^{20}$ cm${}^{\ensuremath{-}3}$ while the ion (atom) temperature is found to be 1050 K, e.g., above the crystallization activation temperature reported in the literature. This low ionization rate and high atom temperature suggest a thermally activated phase transition.