Ultrafast Dynamics of Electron Thermalization in Gold

Time-resolved surface second-harmonic generation (SHG) is used to probe electron relaxation processes in gold following intense laser excitation at 1.55 eV. For the first time, an electron temperature ( T(e)) dependent enhancement in the SHG signal is clearly observed at T(e) above 0.7 eV, which is shown to relate to the thermalization of nonequilibrium hot electrons. Therefore, the relaxation dynamics of the transient nonequilibrium electrons in the high T(e) regime is directly resolved by monitoring the time evolution of the SHG signal.