Raman Spectroscopy on Metallic Surfaces

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Raman spectroscopy is a widespread analytical technique in solid state physics, materials science and many other disciplines. Up to date, a vast number of studies deals with Raman spectroscopy of optical phonons of nonmetallic compounds, such as semiconductors, insulators and molecules. On the contrary, very few investigations of metals can be found in the literature. In fact, metals such as Cu, Ag and Au crystallize in a primitive lattice with a single atom basis and do not possess optical phonon modes. In contrast to bulk systems, at surfaces, even metals with a single atom basis may exhibit optical phonons, which should generally be accessible by Raman spectroscopy. Moreover, the cross section for surface phonon scattering can be greatly enhanced using light at resonance with an optical transition involving surface electronic states, as shown by the authors in a recent publication. A combined experimental and theoretical approach shall now elucidate the underlying scattering mechanism and the resonant enhancement on different metals and surface orientations. In addition, we aim to extend the investigations to adsorbates on surfaces the backfolding of surface phonon branches via adsorbate-induced superstructures. Metal atom dopants are used to explore the possibility of localizing the optical resonances in surface metal alloys for possible applications in surface catalysis. In contrast to conventional Raman scattering or Surface Enhanced Raman Scattering (SERS), surface Resonant Raman scattering (SRRS) allows the investigation of atomically flat metal surfaces. The resonant enhancement occurs via surface electronic states rather than local fields or plasmons as in SERS. SRRS thus bears new potential for high-resolution vibrational spectroscopy at metal surfaces. The scattering mechanism in SRRS will be studied experimentally in two complementary setups at the Technical University of Berlin (TUB) and at the Johannes Kepler University in Linz, Austria (JKU). Corresponding atomistic calculations in the framework of the density functional theory (DFT) and more refined approaches are performed at the Justus Liebig University in Giessen (JLU). In a recent common investigation, the three applicants could demonstrate for the first time Raman activity at the Cu(110) surface. In the present proposal, the Raman activity of different metallic surfaces as well as the underlying mechanisms will be investigated. The experiments at JKU will focus on the bare and adsorbate covered Cu(110) surface, whereas dependencies on the optical wavelength on other substrates like Ag and Au will be studied at TUB. The JLU will provide an atomistic description of the investigated systems and their lattice dynamics to elucidate system specific scattering mechanisms. The common effort of the three involved groups allows to explore the potential of the newly discovered properties of metallic surfaces in surface analysis or, e.g., applications in the field of catalysis.