Ultrafast relaxation dynamics of electronic excitations in noble-metal clusters

Abstract.

We investigate non-equilibrium relaxation processes in optically excited large gold and silver clusters. Time-resolved pump-probe experiments and model calculations show that optical excitation of the clusters by femtosecond laser pulses results in a heating of the electron system, which is followed by electron cooling via phonon emission. The electron heating leads to an enhanced damping of the surface-plasmon resonance in the clusters. This enhanced damping is caused by an enhancement of the Landau damping and electron scattering rates at high electron temperatures. Furthermore, we find that the rate of electron cooling in the clusters changes with electron temperature; this is a consequence of the temperature-dependent specific heat of the conduction electrons. Finally, pump-probe experiments on ellipsoidal silver clusters show that the thermal expansion of the heated clusters triggers mechanical vibrations at the acoustic eigenfrequencies of the clusters.