Ultrafast nonequilibrium dynamics of electrons in metals

Approximate analytic solutions for the athermal relaxation and diffusion of carriers in free-electron metals excited by ultrashort optical pulses are derived. The energy-loss time of the electrons is shown to be similar to tau(0)(1/3)tau(S)(2/3), where tau(0) and tau(S) an characteristic electron-electron and electron-phonon scattering times, By evaluating the average excess energy per excitation we are able to establish an approximate criterion for the time of transition from an athermal electron distribution to a thermalized distribution. We thus demonstrate how one can calculate the evolution of the total energy and number densities of the nonequilibrium electrons at a given point in the metal, linking the athermal to the thermalized regime by interpolation. Quantitative agreement of the theory with some recent experiments on hot electrons in metals is good.