Snapshot Enlightenment
With the help of ultra-fast lasers, scientists at the Max Planck Institute for Quantum Optics and Plasma Physics and researchers at the University of California at Berkeley have succeeded in taking detailed "snapshots" of electron movements on metal surfaces. The behavior of electrons at solid surfaces and interfaces affects, for example, the performance of small transistors in microchips or the chemical reactivity of atoms and molecules in catalysts. The work presented in Science offers the opportunity to study some of the underlying principles in real time. In the experiments, an electron is excited by a short laser pulse - the "pump pulse" - so that it reaches a transition state; a subsequent pulse - the "probe pulse" - emits it into the vacuum. The kinetic energy of the electron and the angle at which it leaves the surface are measured and provide information about its transition state. This technique has been used for several years to determine the lifetime of excited electrons in metals and semiconductors: the emission intensity is measured as a function of the time interval between pump and probe pulses.
In September 1997, researchers from the Max Planck Institute for Quantum Optics and Plasma Physics in Garching and from the University of Erlangen made preparations for considerably more complex measurements (Science, 277, 1480, 1997). They checked the electron dynamics in so-called image-potential states, in which the electron is in a vacuum above a metal surface, but is still weakly bound to it. The German team managed to coherently excite several of these states and generate a wave bundle. For a certain period of time, such a beam can behave like a classical particle, whose distance from the surface is reflected in the strength of the photoemission signal. In the experiment in Garching, the electron was observed moving about 100 atomic distances from the surface and oscillating with a period of 800 femtoseconds.
The work of the Berkeley group (Science, January 23, 1998 issue) also deals with the behavior of electrons in image-potential states, but with a molecular adsorbate layer on the metal surface. Initially, the weakly bound electrons are able to migrate along the surface at will, provided the adsorbate layer is well-ordered. However, by polarizing and shifting individual molecules, the delocalized electrons can be temporarily confined to so-called small “polarons”. The existence of such “self-trapped” electron states was predicted as early as 1933 by the Russian theoretician Landau. For example, they are important for electron transport in conducting polymers or in the course of photosynthetic reactions. The Berkeley experiment has now succeeded in following this trapping process of an electron in real time.
The fact that the two experiments show the dynamics of electrons on surfaces with unprecedented detail is not only important for the physics and chemistry of interfaces. It is expected that other areas will also benefit from this unique ability of time-resolved photoelectron spectroscopy of two-dimensional structures, because ordered layers can now be formed from many substances.
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