Electron and lattice dynamics following optical excitation of metals

New results about relaxation dynamics of optically excited electrons in metals, mostly gold and nickel films, are presented. Emphasis is on electron temperature near the surface as well as on the range of energy transport by ballistic and diffusive electron motion in comparison to the optical penetration depth. The experiments focus on the interval between creation of an electron temperature and the time at which thermal equilibrium between electrons and lattice is reached. Results were obtained by time-resolved linear and second-harmonic reflectivity measurements carried out in pump-probe mode. It is shown that the two-temperature model is well, suited to describe hot electron diffusion in metals and to extract electron-phonon coupling constants from experimental data, provided corrections for ballistic electron motion are incorporated. The electron-phonon coupling constant of gold was found to be independent of film thickness down to 10 nm. For noble metals, probe reflectivities near the interband transition were related to electron temperatures by a proper model for the dielectric function. For transition metals such relation between reflectivity and electron temperature is more difficult. A new pump-pump-probe technique was introduced which allows to study hot electron relaxation by probing the reflectivity in thermal equilibrium between electrons and lattice. Also these results can be well described by the two-temperature model. Finally, the interface sensitivity of the second harmonic was utilized to detect vibrational motion and thermal expansion of ultrathin nickel films on Cu(001). (C) 2000 Elsevier Science B.V. All rights reserved.