Time-resolved two-photon photoemission in combination with the coherent excitation of several quantum states was used to study the ultrafast electron dynamics of imagepotential states on metal surfaces. For a (100) surface of copper, the spectroscopy of quantum beats made previously unresolved high-order states (quantum number n 2 4) experimentally accessible. By exciting electrons close to the vacuum level, electron wave packets could be created and detected that described the uasi-classical periodic motion of weakly bound electrons. They traveled more than 200 1 away from the surface and oscillated back and forth with a period of 800 femtoseconds.Photoelectron spectroscopy has developed into one of the most versatile and successful tools for surface studies. Particularly attractive features of this technique are the high surface sensitivity associated with the low escape depth of the photoelectrons and the capability of angle-resolved photoemission to completely characterize electronic states in energy and momentum space (1). Recently, these features have been combined with ultrafast laser excitation for direct time-domain investigations of electron dynamics at surfaces (2). Here, we demonstrate another facet of this powerful technique, the investigation of coherence phenomena in real time. In contrast to experimental methods that rely merely on intensities, coherent spectroscopies offer the unique capability of accessing not only the amplitudes but also the phases of the wave functions of interest (3). This techniaue dramaticallv increases the amount of infbrmation that dne is able to obtain about the tem~oral evolution of fast Drocesses.In &s report, we discuss thehynamics of image-potential states, that is, the quantized excited states of electrons that exist in front of many metal surfaces (4, 5). Using femtosecond time-resolved two-photon photoemission (2PPE), we observed the interference between the wave functions of neighboring eigenstates and the quasi-classical motion of electron wave packets created by the coherent superposition of several quantum states. Recently, the imaging of the static charge density of related surface electronic (ground) states in real space with the scanning tunneling microscope has attracted considerable interest (6); the present results reveal the dynamical evolution of excited electrons in real time.Image-potential states are conceptually rather simple. An electron at a distance z in front of a conducting metal surface experiences an attractive force F(d = -e2/(2d2 identical to that produced by a positive (mirror image) charge at a distance z inside the metal (Fig. 1A). If the metal has a band gap (in z direction) near the vacuum level E,, = 0, then an electron below E, , may be trapped in the potential well formed by the Coulomb-like attractive image potential V(z) = -e2/4z and the repulsive surface barrier (4). The resulting quantized electronic states form a Rydberg series with energies En converging toward the vacuum energy, where the influence of the surface Dotential o...

We report time-resolved studies of image-potential states on Cu͑100͒ and Ag͑100͒. Femtosecond ultravioletpump and infrared-probe techniques combined with two-photon photoemission have been used to measure relaxation dynamics of nϭ1, 2, and 3 image-potential states. We found the lifetimes of these states on Cu͑100͒ at room temperature to be 40Ϯ6, 120Ϯ15, and 300Ϯ20 fs, respectively. The corresponding values for Ag͑100͒ are 55Ϯ5, 160Ϯ10, and 360Ϯ15 fs.

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