Femtosecond laser pulses irradiating a solid material induce a cascade of processes starting with the excitation of so-called hot electrons and passing through various relaxation processes. Several scattering mechanisms act on different timescales. At sufficiently high energy densities, phase transitions and ultrafast structural dynamics can be induced.

We simulate the dynamics of a large ensemble of excited electrons using complete Boltzmann collision integrals. We consider the excitation of conduction electrons in a metal with visible light. On a femtosecond timescale, the electrons' energy distribution deviates strongly from a Fermi distribution. We extract spectral electron densities within specific
energy windows, and find complex behavior that cannot be matched with a single relaxation time.

We show that electron-electron and electron-phonon scattering mutually influence each other during thermalization. For materials with several electronic systems, e.g. itinerant ferromagnets or dielectrics, we observe that temperatures and partial densities can be independent quantities
on picosecond timescales.