Raman Spectroscopy on Metallic Surfaces

Raman spectroscopy is a widely used technique in physics, chemistry and materials science. In the underlying Raman scattering process, characteristic oscillations in molecules or solids are excited by laser radiation, the frequency and excitation probability of which provide detailed information about the chemical composition and the interaction between individual atoms or molecules. To date, a large number of studies have dealt with the analysis of non-metallic materials, such as semiconductors, insulators or molecules. In contrast, only a few studies of metallic systems are available in the literature. This is because typical metals such as copper, silver and gold form a crystal lattice that contains only a single atom in the unit cell and therefore has no vibrational excitations accessible by Raman spectroscopy. However, the surfaces of solids (even metals with only one atom in the unit cell) exhibit additional vibrational excitations that are more or less localized at the surface. These surface vibrations should in principle be accessible with Raman spectroscopy, as was recently demonstrated in a joint publication by the project partners. The very low excitation probability in the Raman scattering process can be significantly enhanced if a laser energy is used that corresponds to an optical transition between electronic surface states, i.e. if the excitation is surface resonant. A combined experimental and theoretical investigation at the Johannes Kepler University in Linz, the Technical University of Berlin and the Justus Liebig University in Giessen is now to explain the underlying scattering mechanisms for different metals and surface orientations. In addition, surface resonant Raman spectroscopy (SRRS) will be used to characterize the vibrations of adsorbed atoms and molecules on surfaces such as oxygen, xenon and carbon monoxide. Finally, the formation of surface alloys after deposition of metal atoms will be investigated, which is crucial for catalytic applications. In contrast to Surface Enhanced Raman Scattering (SERS), which relies on specially roughened or structured metal surfaces, SRRS allows the investigation of well-defined metallic surfaces that are smooth at the atomic level. SRRS thus offers novel and great potential for high-resolution vibrational spectroscopy of metal surfaces.

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