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"Femtosecond Structural Dynamics of the Prototypical Si(001) Surface"
G. Le Lay, C.-C. Fu, M. Weissmann and A. Saúl
Proc. of the "TDR XFEL Workshop Series: Surfaces, Interfaces and Nanomaterials", February 27, 2001.
Scientific Case of the X-ray Free Electron Laser (XFEL) at DESY, Hamburg (2001) 30-34
Physical, chemical and biological processes are by nature dynamic; many of these processes at the frontiers of research take place on the time scale of molecular vibrations, typically around 100 femtoseconds (fs). X-ray experiments to study structural changes that occur on such a time scale are an emerging area of research in condensed matter physics. At the present time, at third generation synchrotron radiation facilities, one can, for example, slice the stored 30 picosecond (ps) electron bunch to generate 300 fs X-ray pulses. Other methods, like, e.g., relativistic Thomson scattering, have also been used recently to develop subpicosecond sources of hard X-rays. Ultra-short visible-pump, X-ray diffraction probe experiments have already been successfully carried out. A nice example concerns the detection of nonthermal melting in nominally 160 nm thick germanium thin films by ultra-fast, time-resolved, 1.54 Å X-ray diffraction. The 1 Å X-ray Free Electron Laser (FEL) in project at DESY will give a fantastic impetus to such investigations due to the enormous gain in brilliance, the full coherence and still the reduction to 100 fs of the length of the pulses, compared to the just mentioned ultra-fast Xray sources. With typically 1012 photons per bunch of Self Amplified Spontaneous Emission (SASE) FEL radiation, the study of dynamical processes occurring at a femtosecond time scale, at the very surface itself, will become possible. This is indeed a crucial issue in many different areas such as, obviously, heterogeneous catalysis, crystal growth etc.. We envisage here a fundamental study that could serve as a benchmark case for many other experiments. We propose the structural observation of the dynamics of a semiconductor surface of paramount importance: the silicon (100) surface itself. After a brief description of the present knowledge of the different structures and phase transitions occurring at this surface we will report on new molecular dynamics simulations, currently in progress, that give deeper insights into the peculiar behavior of this surface and already demonstrate the feasibility of the planned study, before describing the structural measurements we could perform.
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