Evolution of the transient electron energy distribution ρ e ( E ) in XFEL-irradiated Cu with pulse intensity I = 5 x 10 12 W cm -2. The curves represent the electron distributions at various time instants with time zero corresponding to the maximum of the 15 fs FWHM Gaussian pulse. ρ e is defined by ∫ ρ e ( E )d E = 〈 Z 〉 where 〈 Z 〉 denotes the transient ionization degree per atom, that is, the number of free electrons above the vacuum level divided by the total number of atoms. The black dashed curves show the MaxwellBoltzmann (MB) distributions with a kinetic temperature equal to the average electron kinetic energy at 20 fs: 2.2 eV. The broad peak with a substructure at around 250 eV, as indicated by a blue arrow, corresponds to the secondary electron emission during the collisional ionization of neutral Cu and Cu + ground states. The sharp peak at 943 eV, as indicated by a red arrow, is due to the Auger decay of the 2 p 3/2 core hole.
