Studies of current dynamics in solids have been hindered by insufficiently brief trigger signals and electronic detection speeds. By combining a coherent control scheme with photoelectron spectroscopy, we generated and detected lateral electron currents at a metal surface on a femtosecond time scale with a contact-free experimental setup. We used coherent optical excitation at the light frequencies omega(a) and omega(a)/2 to induce the current, whose direction was controlled by the relative phase between the phase-locked laser excitation pulses. Time- and angle-resolved photoelectron spectroscopy afforded a direct image of the momentum distribution of the excited electrons as a function of time. For the first (n = 1) image-potential state of Cu(100), we found a decay time of 10 femtoseconds, attributable to electron scattering with steps and surface defects.

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We demonstrate the existence of buried image-potential states at the interface between thick Ar films and a Cu(100) substrate. The electron dynamics of these solid-solid interface states, energetically located above the vacuum level in the band gaps of both materials, could be investigated with time-resolved two-photon photoemission for an Ar layer thickness up to 200 A. Relaxation on time scales between 40 and 200 fs occurs via two distinct channels, resonant tunneling through the insulating layer into the vacuum and electron-hole pair decay in the metal.

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During the last decade, great progress has been made by investigating electron transport through single molecules, metallic point contacts, or chains of single atoms by using scanning tunneling microscopy or break junctions [1,2]. Many of these works have focused on the coherent regime where inelastic scattering of the electrons due to vibronic or electronic excitation within the junction can be neglected. In this regime, nanoscale wires show no ohmic behavior and a quantization of the conductance has been observed as can be described by the Landauer theory [3]. While these experiments observe electron transport on the atomic scale, information about the dynamics of electron transport on the timescale of the relevant scattering mechanisms is scarce. The microscopic understanding of the mechanisms of electron transport, however, is not only of great interest for the development of new small-scale electronic devices, the connection of the electric conductivity s of a material with the microscopic scattering processes of individual charge carriers is also a fundamental issue, central to solid state physics [4]. This question goes back to Paul Drude [5] who has connected the electric conductivity s of a metal to an empirical scattering time t of free electrons that are accelerated in an external electric field, resulting in the well known Drude formula s ¼ e 2 n e t=m e , where e, m e , and n e are the electron charge, mass and density, respectively. Even if this model is oversimplified as it does not consider the quantum nature and the many-body aspects of electronelectron interaction, it gives the correct order of magnitude for the timescale of electron scattering processes. For Cu at room temperature, for example, it gives a scattering time of 27 fs [4], which shows that usual conductivity measurements cannot resolve the individual scattering events because available electronic equipment can neither produce trigger signals nor detect transients that are shorter than tens of picoseconds.Recently, we have introduced a new experimental technique that is capable of accessing the dynamics of electrical currents on the femtosecond timescale [6]. This contact-free technique combines the pure optical generation of electric