Raman Spectroscopy on Metallic Surfaces

拉曼光谱是固体物理、材料科学和许多其他学科中广泛应用的分析技术。迄今为止，非金属化合物（如半导体、绝缘体和分子）光学声子的拉曼光谱研究已经取得了大量的成果。相反，在文献中很少能找到对金属的研究。事实上，诸如Cu、Ag和Au的金属在具有单原子基础的原始晶格中结晶，并且不具有光学声子模式。与体相系统相反，在表面处，甚至具有单原子基础的金属也可以表现出光学声子，其通常应该可以通过拉曼光谱法获得。此外，如作者在最近的出版物中所示，使用与涉及表面电子态的光学跃迁共振的光可以大大增强表面声子散射的横截面。结合实验和理论的方法，现在将阐明潜在的散射机制和不同的金属和表面取向的共振增强。此外，我们的目标是扩大调查吸附在表面上的表面声子分支的回折通过吸附诱导的超结构。金属原子掺杂剂被用来探索在表面金属合金中定位光学共振的可能性，以用于表面催化的可能应用。与传统的拉曼散射或表面增强拉曼散射（Sers）相比，表面共振拉曼散射（SRRS）允许研究原子级平坦的金属表面。共振增强通过表面电子态而不是如Sers中的局部场或等离子体发生。因此，SRRS在金属表面的高分辨率振动光谱具有新的潜力。SRRS中的散射机制将在柏林技术大学（TUB）和奥地利林茨的约翰内斯开普勒大学（JKU）的两个互补装置中进行实验研究。在密度泛函理论（DFT）和更精细的方法框架内的相应原子计算在吉森的Justus Liebig大学（JLU）进行。在最近的共同研究中，三个申请人可以首次证明在Cu（110）表面的拉曼活性。在本提案中，不同的金属表面的拉曼活性以及潜在的机制将被调查。JKU的实验将集中在裸露的和吸附物覆盖的Cu（110）表面上，而TUB将研究对其他基底（如Ag和Au）上光波长的依赖性。该JLU将提供一个原子的描述所研究的系统和它们的晶格动力学阐明系统特定的散射机制。三个相关小组的共同努力允许探索表面分析中新发现的金属表面特性的潜力，例如，在催化领域的应用。

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.