At the FERMI FEL, beamline EIS-TIMEX, a novel approach combining FEL and fs-laser radiation has been developed for generating liquid carbon under controlled conditions and monitoring its properties of at the atomic scale. The method has been put to the test depositing a huge amount (5 eV/atom, 40 MJ/kg) of optical energy delivered by an ultrashort laser pulse (less than 100 fs, 10-13 s) into a self-standing amorphous carbon foil (a-C, thickness about 80 nm) and subsequently probing the excited sample volume with the FEL pulse varying both the FEL photon energy across the C K-edge (~ 283 eV) and delay between FEL and laser. A time-resolved x-ray absorption spectroscopy (tr-XAS, Fig. 2a) has been obtained of l-C with a record time resolution of less than 100 fs.
This method allowed researchers to monitor the formation of the liquid carbon phase at a temperature of 14200 K and pressure of 0.5 Mbar occurring in about 300 fs after absorption of the laser pump pulse as an effect of the constant volume (isochoric) heating of the carbon sample.
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Image: Artistic image illustrating the ultrafast laser-heating process used to generate liquid carbon in the laboratory. Illustration: Emiliano Principi.