Ultrafast processes in Ag and Au: A Monte Carlo study

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Abstract
Monte Carlo simulations of ultrafast electron processes in Ag and Au have been used to analyze transient reflectivity as well as two-photon photoemission experiments. The model consisted of an electron Fermi gas coupled to longitudinal acoustic phonons. A laser pulse of finite width excited the electrons, after which the development of the distribution function was followed during 3–5 ps. In the electron-phonon coupling we used the full q dependence of the scattering together with a semirealistic phonon dispersion relation. The strength of the electron-phonon scattering is governed by the deformation potential. Its value was fixed to low-fluence transient reflection data. Also hot-phonon effects were included. For the electron-electron scattering we employed a Coulomb interaction screened according to the Thomas-Fermi prescription. We accounted for exchange effects in the total scattering probability. In a later stage we varied the screening. We analyzed a variety of data for Ag and Au. A good description of the temperature dependence of the transient reflectivity for various excitation powers could be obtained. Also the time evolution of the Fermi surface showed fair agreement with the experiment. In the case of Ag the lifetime of an electron above the Fermi sea was predicted correctly. For Au, however, it was necessary to increase the screening to obtain the correct lifetimes of electrons excited above the Fermi sea. Using this adjusted screening the description of the other experiments no longer was appropriate. Finally also the resistivity due to electron-phonon scattering was predicted quite well using the deformation potential extracted from the ultrafast reflectivity experiments.