Phase transition of carbon induced by X-ray free electron laser

Extremely strong pulses from X-ray free electron laser (XFEL) can change the material structure. Recently, scientists at LCLS (Linac Coherent Light Source), Stanford, USA, have reported the amorphous to crystalline phase transition of carbon by femtosecond 830 eV XFEL beam. The research group employed atomic force microscopy, photoelectron microscopy, and micro-Raman spectroscopy to discuss the change of the sp²/sp³ ratio (graphitization), as well as the change of local order of the irradiated sample area. It was found that the phase transition threshold fluence is 282 ± 11 mJ/cm², and also the transition is mainly due to thermal activation rather than a non-thermal mechanism such as ionization etc. For more information, see the paper, "Amorphous to crystalline phase transition in carbon induced by intense femtosecond x-ray free-electron laser pulses", J. Gaudin et al., Phys. Rev. B86, 024103 (2012).