物構研談話会（14-12）Studies on Surface Structure using Fast Atom Diffraction (FAD)
|講演者||Professor Dr. Helmut Winter (Institute of Physics, Humboldt University)|
Some years ago, pronounced diffraction effects for grazing scattering of fast light atoms and molecules with energies up to some keV under axial surface channeling were observed. The rich diffraction patterns provide information on the inter-atomic spacings between axial surface channels and on the corrugation of the interaction potential. The latter effect can be used to study the structure of surfaces with fast atoms via interferometric techniques. The new method shows similarities to RHEED/RHEPD and thermal He atom scattering (HAS), but has a number of advantages compared to the latter as simple tuning of the projectile energy (de Broglie wavelength), an orders of magnitude more efficient detection of scattered projectiles as well as a simple and cost-effective setup.
The quantum coherence in the scattering process is preserved by specific features of surface channeling which is investigated in detail via the coincident detection of the diffraction patterns with the energy loss of scattered atoms. It turns out that the suppression of electronic excitations owing to the band gap of insulator surfaces plays a key role for coherent scattering and the application of Fast Atom Diffraction (FAD) in surface science. Whereas for H atoms the electron transfer is important for decoherence in the scattering event, for He atoms the energy transfer to lattice atoms dominates the loss in coherence. Quantum decoherence determines the projectile energies and incidence angles where FAD can be applied.
Recent work using FAD for studies on the structure of surfaces has been focused on the longitudinal coherence which is observed for the formation of superstructures of sapphire surfaces. The presence of longitudinal coherence enhances the resolution of the method (transfer width) by more than one order of magnitude. Furthermore it could be shown that FAD is a powerful method to investigate ordered films formed by organic molecules as the amino acid alanine adsorbed on a Cu(110) surface.