DIRECT MEASUREMENTS OF THE TRANSPORT OF NONEQUILIBRIUM ELECTRONS IN GOLD-FILMS WITH DIFFERENT CRYSTAL-STRUCTURES

The transport of femtosecond-laser-excited nonequilibrium electrons ac ross polycrystalline and single-crystalline gold films has been invest igated through time-of-flight measurements. The thicknesses of the fil ms range from 25 to 400 nm. Ballistic electrons as well as electrons i nteracting with other electrons and/or with the lattice have been obse rved. The ballistic component dominates the transport in the thinner f ilms, whereas the interactive transport mechanism is dominant at the u pper end of the thickness range. A slower effective velocity of the in teractive component is observed in the polycrystalline samples, and is assumed to arise from the presence of grain boundaries. The reflectio n coefficient of excited electrons at the grain boundaries is extracte d from the experiment and is estimated to be r congruent to 0.12. The experiment also suggests that thermal equilibrium among the excited el ectrons is not fully established in the first similar to 500 fs after excitation.