Using ultrafast lasers, Max Planck scientists and a research group from the Chemistry Department of the University of California at Berkeley succeeded in taking detailed 'snap shots' of electron motion at metal surfaces. The behavior of electrons at solid surfaces and interfaces controls diverse phenomena, such as the performance of small transistors in microchips or the chemical reactivity of atoms and molecules at catalysts. The work reported in Science demonstrates the ability to investigate some of the underlying fundamentals in real time.
In the experiments a short pump laser pulse excites an electron into an intermediate state, a subsequent probe pulse emits it into the vacuum. The kinetic energy as well as the angle at which the electron leaves the surface are measured and yield the information about its intermediate state. For several years this technique has been used to determine the lifetime of excited electrons in metals and semiconductors by recording the two-photon photoemission intensity as a function of the delay time between pump and probe pulses.
In September 1997, researchers from the Max Planck Institutes for Quantum Optics and Plasma Physics in Garching and from the University of Erlangen in Germany set the stage for considerably refined measurements (Science, 277, 1480, 1997). They investigated electron dynamics in so-called image-potential states, wherein the electron is located in the vacuum above a metal surface, but is still weakly bound to it. The German team succeeded in coherently exciting several of these states and created a wave packet. For a certain period of time such a wave packet can behave like a classical particle whose distance from the surfaces is reflected in the strength of the photoemission signal. In the Garching experiment the electron was observed to move about 100 atomic distances away from the surface and oscillated back and forth with a period of 800 femtoseconds.
The work of the Berkeley group (Science, 279, 202, 1998) also concerns the behavior of electrons in image-potential states but with a molecular adsorbate layer on top of the metal surface. Initially the weakly bound electrons are able to move freely along the surface if the adsorbate layer is well ordered. However, by polarizing and displacing individual molecules the delocalized electrons can become momentarily confined in so-called small polarons. The existence of such self-trapped electronic states has already been predicted by the Russian theorist Landau in 1933. They are important, e.g. for electron transport in conductive polymers or during photosynthesis reactions. With the Berkeley experiment it has now become possible to follow this trapping process of an electron in real time.
The fact that the two experiments revealed the dynamics of electrons at surfaces with unprecedented detail is significant not only for the physics and chemistry of interfaces. It is expected that also other fields will benefit from the unique capabilities of time-resolved photoelectron spectroscopy of two-dimensional structures because today ordered layers can be prepared from many substances (Science, 279, 190, 1998).
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Science