Phys. Rev. X 8, 031034 (2018)

Exactly what happens when an intense pulse from an X-ray free-electron laser (FEL) strikes a nanoscale target has now been revealed by an international collaboration of scientists from Japan, Finland, Estonia, France, Romania, Germany and China. It is well known that a plasma is formed, but the timescale and details of its birth and evolution have to date remained hidden, largely because of the temporal jitters on the order of 1 ps that exist between the XFEL pump pulses that creates the plasma and the near-infrared (NIR) probe pulses that are used to investigate it. Now, this issue has been overcome and scientists have succeeded in monitoring the birth of a nanoplasma formation in Xe clusters. The experiments were performed at the XFEL facility in Japan, SACLA, using 5.5-keV X-ray pulses with a bandwidth of 40 eV, duration of less than 10 fs and a peak fluence of 30 μJ μm–2, and NIR pulses (central wavelength of 800 nm and pulse duration of 32 fs) with a peak fluence of 15 nJ μm–2. The experiments show that the plasma formation is accompanied by the generation of excited states, with an ultrafast population (~12 fs) followed by a slower depopulation (~250 fs) of excited states.