We report a time-resolved study of the relaxation dynamics of Al films excited by ultrashort intense free-electron laser (FEL) extreme-ultraviolet pulses. The system response was measured through a pump-probe detection scheme, in which an intense FEL pulse
tuned around the Al L2,3 edge (72.5 eV) acted as the pump, while a time-delayed ultrafast pulse probed the near-infrared (NIR) reflectivity of the Al film. Remarkably, following the intense FEL excitation, the reflectivity of the film exhibited no detectable variation for hundreds of fs.
Following this latency time, sizable reflectivity changes were observed.
Exploiting recent theoretical calculations of the EUV-excited electron dynamics [N. Medvedev et al., Phys. Rev. Lett. 107 165003 (2011)], the delayed NIR-reflectivity evolution is interpreted invoking the formation of very-long-living non-thermal hot electron distributions in Al after exposure to intense EUV pulses. Our data represent the first evidence in the time domain of such an intriguing behaviour.