EFFECT OF A NONTHERMAL ELECTRON-DISTRIBUTION ON THE ELECTRON-PHONON ENERGY RELAXATION PROCESS IN NOBLE-METALS

We show experimentally that the electron distribution of a laser-heated metal is a nonthermal distribution on the time scale of the electron-phonon (e-ph) energy relaxation time tau(E). We measured tau(E) in Ag and Au films as a function of lattice temperature (10-300 K) and laser-energy density (0.3-1.3 J cm-3), combining femtosecond optical transient-reflection techniques with the surface-plasmon polariton resonance. Calculations support the experimental result that the nonthermal distribution leads to a slower e-ph energy relaxation process below room temperature, compared to the thermalized limit.