Ultrafast Electron Dynamics at Cu(111): Response of an Electron Gas to Optical Excitation

Abstract

Time-resolved two-photon photoemission is used to directly investigate the electron dynamics at a Cu(111) surface with 60 fs laser pulses. We find that the time evolution of the photoexcited electron population in the first image state can be described only by solving the optical Bloch equations to properly account for coherence in the excitation process. Our experiments also provide evidence that the dynamics of photoexcited bulk electrons is strongly influenced by hot electron cascades and that the initial relaxation rates are in agreement with Fermi liquid theory. PACS numbers: 78.47.+p, 73.25.+i Ultrafast lasers have been extensively used to study the relaxation and recombination dynamics of excited bulk carriers in semiconductors [1-3] and nonequilibrium processes in laser-heated metals [4,5]. In contrast to techniques based on dynamic changes of macroscopic optical constants (e.g., transmission, reflection) time-resolved photoemission (TRPE) allows a direct measurement of the temporal evolution of the photoexcited electron distribution. In metals, where the initial relaxation dynamics is governed by ultrafast electron-electron ͑e-e͒ scattering, TRPE experiment have shown that complete thermalization of the nascent hot electrons to a Fermi distribution occurs on a time scale comparable to the electron-phonon ͑e-ph͒ relaxation time ͑ϳ1 ps͒ [5]. Image-potential states at metal surfaces provide an ideal system to study an excited electron gas and its coupling to a continuum of substrate excitations [6,7]. Time-resolved studies of the image state dynamics, however, provide a challenge due to the fast decay of the excitation on the order of few to tens of femtoseconds depending on their binding energy with respect to the bulk band structure.For coherent excitation of an electron gas with nearly transform limited laser pulses the rate of excitation is no longer given by Fermi's golden rule and cannot be described by simple rate equations. In order to properly account for coherent excitation as well as for energy relaxation and dephasing the optical Bloch equations must be solved [8]. This treatment reveals that the rate of excitation is not the highest when the field strength of the laser pulse reaches its maximum but rather when the pulse intensity decreases again. In this Letter we show that a precise measurement of the delay between the time profile of the pulse and the evolution of the excited state population, which critically depends on the energy relaxation time ͑T 1 ͒, allows us to analyze lifetimes which are considerably shorter than the laser pulse duration.We use time-resolved two-photon photoemission (2PPE) to investigate the response of an electron gas to femtosecond excitation at low excitation densities. Our experiments show that the transient population of photoexcited electrons in the ͑n 1͒ image state on Cu(111) is retarded by ϳ17 fs with respect to the time response of a two-photon process from the occupied surface state via a virtual intermediate state in the sp-band gap. The observed respon...

Decay of electronic excitations at metal surfaces

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Distinguishing attosecond electron–electron scattering and screening in transition metals

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Experimental time-resolved photoemission andab initiostudy of lifetimes of excited electrons in Mo and Rh

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