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...

Time-resolved fluorescence measurements of single-walled carbon nanotubes (SWNTs) reveal rapid electron-hole pair annihilation when multiple electron-hole pairs are present in a nanotube. The process can be understood as Auger recombination with a rate of ϳ1 ps −1 for just two electron-hole pairs in a 380-nm long SWNT. This efficient nonradiative recombination reflects the strong carrier-carrier interactions in the quasione-dimensional SWNTs. In addition to affecting nanotube fluorescence, Auger recombination imposes limitations on the sustained electron-hole density achievable within a SWNT.

Electrons tunneling from a scanning tunneling microscope tip to individual CO molecules on Cu(111) can cause their hopping from the surface to the tip if the bias exceeds a threshold of 2.4 V. Polarization-and time-resolved two-photon photoemission identifies the underlying elementary process as intermediate population of a CO 2p ‫ء‬ -derived level, which exhibits an ultrashort lifetime of 0.8 -5 fs. From an isotope effect of 2.7 10.3 20.5 it can be calculated that ഠ0.05% of the tunneling current transiently occupies this level while a desorption of the excited molecule occurs only in 5 3 10 29 of the cases.[S0031-9007(98)

Time-resolved two-photon photoemission ͑2PPE͒ at various photon energies is used to investigate the relaxation dynamics of hot electrons in Cu͑111͒, applying auto-and cross-correlation techniques. The relaxation times vary from 250 fs at 0.1 eV above the Fermi level to 20 fs at 2 eV and show a strong wavelength dependence in the vicinity of the d-band feature in the 2PPE spectra. Electrons not directly excited from the d band exhibit a much longer relaxation time than d-band electrons excited to the same intermediate-state energy. We attribute these apparently longer lifetimes to a delayed electron generation via Auger decay of d-band holes. Based on a band structure calculation, a simulation of the ballistic transport effect and its implication on the observed electron relaxation dynamics is presented for the three low-index copper surfaces. These observations suggest that d-band holes have a substantially longer lifetime than excited sp-band electrons of the corresponding excitation energy. ͓S0163-1829͑98͒03720-5͔

Lifetimes of electrons in the nϭ1 and nϭ2 image states on Cu͑111͒ are studied with femtosecond timeresolved photoemission. Adsorption of one monolayer of Xe results in a pronounced increase of the imagestate lifetime, which for the nϭ1 state changes from 18Ϯ5 fs at clean Cu͑111͒ to 75Ϯ15 fs at the Xe-covered surface. The slower relaxation rate induced by the Xe layer is attributed to a reduced overlap of the image-state wave function with bulk states. A density-matrix calculation reveals the importance of dephasing in the excitation process. ͓S0163-1829͑96͒51032-5͔