Tracking the time-evolution of the electron distribution function in Copper by femtosecond broadband optical spectroscopy.

Multi-temperature models are nowadays often used to quantify the ultrafast electron-phonon (boson) relaxations and coupling strengths in advanced quantum solids. To test their applicability we study the time evolution of the electron distribution function, f(E), in Cu over large range of excitation densities using broadband time-resolved optical spectroscopy. Following intraband optical excitation, f(E) is found to be athermal over several 100 fs, while substantial part of the absorbed energy already being transferred to the lattice. We show, however, that the electron-phonon coupling constant can still be obtained using the two-temperature model analysis, provided that the data are analyzed over the time-window, when the electrons are already quasi thermal, and the electronic temperature is determined experimentally.