Electron diffusion in metals studied by picosecond ultrasonics

We have studied the ultrasonic pulses that are generated when a picosecond light pulse is absorbed in a metallic film. The pulse shape is influenced by the distance that the hot electrons diffuse before losing their energy to the lattice. We show that this method can be used to test theories of the electron-phonon and electron-electron scattering rates. The results are in excellent agreement with a calculation in which the nonthermal character of the electron distribution is taken into account.