RELAXATION OF FEMTOSECOND PHOTOEXCITED ELECTRONS IN A METALLIC SAMPLE

A model calculation is given for the energy relaxation of a nonequilibrium distribution of hot electrons prepared in a metallic sample that has been subjected to homogeneous photo-excitation by a femtosecond laser pulse. The model assumes that the photoexcitation creates two interpenetrating electronic subsystems, initially comprising of a dilute energy-wise higher-lying nondegenerate hot electron subsystem, and a relatively dense, lower-lying electron subsystem which is degenerate. The relaxation process is taken to be dominated by the electron–(multi)phonon interaction, resulting in a quasi-continuous electron energy loss to the lattice bath. A novel physical feature of the kinetics considered here is the slowing down of the electron–phonon relaxation in the vicinity of the Fermi energy of the degenerate subsystem, due to the fermionic blocking of the interaction phase space. This leads to a peaking of the calculated hot electron distribution at the Fermi energy. This feature, as well as the entire evolution of the hot electron distribution, may be time-resolved by a pump-probe study. The model is particularly applicable to disordered metallic systems with low electron concentrations and strong electron–phonon coupling, and that are at high temperatures, where the usual two-temperature model^1–3 may not be appropriate.