We report the direct observation of the thermalization of electrons in gold following 180 fs optical pulse excitation. The evolution of the electron energy distribution from the nascent (as photoexcited) to a hot Fermi-Dirac distribution was measured by time-resolved photoemission spectroscopy. Depending on the excitation density, thermalization times as long as =1 ps were observed. A model incorporating both electron-electron and electron-phonon scattering, and using Fermi-liquid theory to properly account for screening is found to reproduce the main features of the experiment.Electron-electron (e-e) scattering in metals has usually been studied by transport' measurements. The contribution of e-e scattering to resistance can only be observed at low temperature, because above the Debye temperature electron-phonon (e-p) scattering completely dominates the resistivity.According to Landau s Fermi-liquid theory, the resistance due to e-e scattering is p, , = AT, where T is the temperature and A is a constant. However, even at low temperature, extraction of p, , from the measured resistivity is complicated by electron-phonon (e-p) and defect scattering. ' Observation of the thermalization of electrons excited by ultrafast optical pulses provides an alternative means to study e-e scattering. The relaxation of an optically excited, non-Fermi-Dirac distribution to a hot Fermi-Dirac distribution is mainly through e-e scattering due to the large momentum exchange and large phase space available for the process which involves quasiparticle energies in the range of an electron volt.In this paper, we report the first direct measurement of the thermalization process in an optically excited metal. We are able to observe the nascent (as photoexcited) electron energy distribution, and the time evolution from the nascent distribution to a Fermi-Dirac distribution. The thermalization process is found to take up to -1 ps for low optical excitation levels, and proceeds more rapidly for higher optical excitation levels. Because thermalization and electron-phonon energy relaxation occur on similar time scales {on the order of ps), we find that even in this regime it is necessary to simultaneously include both e-e and e-p scattering to fully understand the dynamics. A model based on the Boltzmann transport equation under the relaxation-time approximation is pro-t=0 fs 130 fs 400 fs 670 fs 1300 fs 0.1 0 C ENERGY (eV) FIG. 1. Electron energy distribution function vs energy with 120 pJ/cm absorbed laser fluence at five time delays. The dashed line is the best Fermi-Dirac fit and the corresponding electron temperature T,, is shown. The vertical scale is in units of the density of states.posed to explain the experiment. Fermi-liquid theory is used to properly account for Coulomb screening. Time-resolved photoemission spectroscopy was used to measure the time evolution of the electron energy distribution following ultrashort laser pulse excitation of a gold sample. The sample was a room temperature 300-A-thick gold film held in vacuum at 5X10...

The electron-energy distribution in a gold film was measured with -700 fs time-resolved photoemission spectroscopy following laser heating by a 400 fs visible laser pulse. The measured distribution can be fitted by the Fermi-Dirac function at an elevated temperature except within 800 fs of the heating pulse (time-resolution limited), when a reproducible departure is observed. As a result, the relaxation of nonequilibrium electrons was found to be inadequately described by the standard electron-phonon coupling model. PACS numbers: 78.47.+pThe fact that the electronic heat capacity of metals is 1 to 2 orders of magnitude smaller than the lattice heat capacity has led to many investigations of nonequilibrium phenomena in metals with subpicosecond lasers. Model calculations suggest that it should be possible to heat the electron gas to a temperature T e of up to several thousand K for a few ps while keeping the lattice temperature Ti relatively cold [1,2]. Observing the subsequent equilibration of the electronic system with the lattice allows one to directly study electron-phonon coupling under various and unusual conditions [3]. Detailed understanding of the electron-electron (e-e) and electron-phonon relaxation mechanisms should also provide greater insight into chemical reactions [4] and phase transitions [5] induced by ultrashort laser pulses. Several groups have undertaken such investigations by relating dynamic changes in the optical constants (reflectivity, transmissivity) to relative changes in electronic temperature [6-10]. However, no direct measurement of electron temperature has been reported so far. More importantly, the fact that T e is a valid concept only if the electron gas is fully thermalized has often been ignored. Direct measurement of the dynamics of the electron distribution by photoemission spectroscopy provides a much more complete picture of the mechanisms of relaxation of such highly nonequilibrium systems. Not only are difficulties of relating the dynamic changes of the optical constants to the electron temperature removed but the direct measurement of the energy distribution allows for the experimental investigation of the usual implicit assumption that the hot electron gas is immediately and fully thermalized.In this Letter we report the direct measurement of the electron-energy-distribution dynamics in gold films using subpicosecond laser photoemission spectroscopy. A 674-nm-wavelength (1.84 eV photon energy) pump pulse of 400-fs duration was used to excite a 300-A-thick polycrystalline gold film. The heating pulse fluence varied from 0.4 to 1.6 mJ/cm 2 and 15% of the light was absorbed. The laser system consists of a dual-jet synchronously pumped dye laser amplified to 200 juJ/pulse by a 100-Hz excimer-laser-pumped dye amplifier system. The 225-nm probe pulse (5.52 eV photon energy) was produced by first frequency doubling the 674-nm amplified dye laser output in a potassium-dihydrogenphosphate (KDP) crystal and subsequently frequency mixing the 674-nm radiation with the 337-nm secondh...

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