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...

Tunable coherent ultraviolet radiation has been used to excite selectively the n =3 level in hydrogen and deuterium atoms via two-photon absorption from the ground state. The resulting Balmer-a fluorescence at 656 nm was observed as well as resonant three-photon ionization.This work demonstrates several advantages over other techniques for selective detection of neutral H and D atoms, including three-dimensional spatial resolution and a remote monitoring capability afforded by the use of laser-induced-fluorescence detection.We report the first observation of two-photon excitation from the n =1 to the n =3 level in hydrogen atoms. The process was detected by both laserinduced fluorescence from the 656-nm Balmer-o. transition and two-photon resonant three-photon ionization. Both techniques were highly sensitive, and provided high spatial and temporal resolution. While two-photon resonant three-photon ionization of atomic hydrogen via the n = 2 state has been previously demonstrated as a sensitive detection scheme, ' the use of the Balmer-e fluorescence diagnostic provides an attractive alternative in many experimental situations. In addition, our basic technique opens up new possibilities for laser spectroscopy in atomic hydrogen, a subject of great fundamental interest.A schematic of the excitation, fluorescence, and ionization processes is shown in Fig. 1. Two photons are absorbed to excite the 3s and 3d states. An examination of the relevant transition matrix elements3 shows that excitation of 3d dominates over 3s by at least an order of magnitude for our case of %nearly polarized light. As discussed below, two-photon excitation was observed with either A. i = A. 2=205 nm or A. i =193 nm and F2=218 Om. The excited atoms may then decay to the n =2 level via fluorescent emission at 656 nm, or they may be ionized by absorption of an additional ultraviolet photon.The excitation source used for these experiments was a tunable argon fluoride excimer laser, 4 which was frequency shifted via stimulated Raman scattering in D2. The fundamental laser wavelength was 180-150 LLJ O z 90 O K CQ~S IGNAL 656.3nm h~2 0 60-30-0-HOR D FIG. 1. Schematic energy-level diagram of atomic hydrogen showing the excitation, ionization, and fluorescence processes involved in this experiment. 24 612

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Subpicosecond laser pulses have been applied to investigate multiphoton photoemission from singlecrystal Pt(1 1 1) surfaces. Image-potential surface states on the Pt(111) surface give rise to sharp resonant enhancements in the multiphoton photoemission spectrum which allow for unambiguous identification of above-threshold peaks.The phenomenon of above-threshold ionization (ATI) has been extensively studied in atomic and molecular photoionization, but how the optical physics which leads to ATI (ponderomotive effect) influences the photoelectric effect from solids has received comparably little attention.Two studies of above-threshold photoemission (ATP) from polycrystalline metal surfaces have recently been reported with moderate' or poor energy resolution. In both experiments, ATP effects were identified via the detection of electrons with energy greater than the minimum multiphoton photoeff'ect energy, E;"=nohv -+, where no is the minimum number of photons of energy hv necessary to overcome the work function N. However, the spectacular series of peaks, separated by hv, characteristic of ATI from atoms was not observed at all in Ref. 2, and only suggested in the spectra reported in Ref. 1, due to poor energy resolution.In this experiment we study the intensity dependence of the multiphoton photoemission spectrum from the clean, single-crystal Pt(111) surface. The unique feature of this sample is that it is known to possess a special class of surface electronic states known as image-potential states. These states arise due to the fact that an electron outside a metal surface can be bound to its image charge, forming a Rydberg series of states converging on the vacuum level. These states have the character of a one-dimensional hydrogen atom with a potential in the z direction of V(z) =e /4z, hence a binding energy for the first excited state of approximately -, ', Ry=0.85 eV, where Ry is the Rydberg constant. Measured image-state binding energies for a variety of materials and crystalline orientations actually range ' from 0.4 to 1.0 eV due to the departure of the crystal dielectric constant from unity as well as the details of the electron wave-function phase shift upon reflection at the metal-surface-potential discontinuity.The binding energy of the n =1 (first excited) image state on Pt(111) has been reported as 0.63 eV.When present, image states have a dramatic influence on multiphoton photoemission spectra. They provide a very strong intermediate-state resonant enhancement, giving rise to exceptionally sharp features in the photoelectron spectrum, even when there are no sharp initial-state features below the metal Fermi level. This property has made it possible for us to clearly and unambiguously identify sharp peaks in the photoemission spectrum with energy greater than E;"asarising due to ATP. Furthermore, B dN dE (ar b. unit. s) 2.0 2.4 2.8 3.2 3.6 0 ]. 2 3 4 ENERGY(eV) FIG. I. Multiphoton photoemission spectrum from a Pt (111) single-crystal surface. The laser intensity was 15 GW/cm'. due to the extreme...