Nonequilibrium electron interactions in metal films

Summary form only given.Femtosecond spectroscopy has been largely exploited in metal films to investigate electron interaction processes, with a special emphasis on the internal and/or external electron gas thermalization due to electron-electron and electron-phonon scattering. The interplay between these processes for strongly nonequilibrium distributions has been evoked but not directly demonstrated. We have investigated this effect using a femtosecond pump-probe technique with 20-fs pulses. Measurements were performed by probing the changes of the optical properties of a metal film far from the interband transitions. This permits investigation of the average electron properties and of their dependence on the electron distribution. The results clearly demonstrate acceleration of both the energy losses of the electron gas to the lattice and of the average electron-electron scattering, during the early stages of the nonequilibrium electron relaxation. Similar results were also obtained in metal nanoparticles systems.