Raman Spectroscopy on Metallic Surfaces

拉曼光谱是一种广泛应用于固体物理、材料科学等诸多学科的分析技术。到目前为止，大量的研究涉及非金属化合物的光学声子的拉曼光谱，如半导体、绝缘体和分子。相反，在文献中很少能找到对金属的研究。事实上，铜、银和金等金属是以单原子为基元的原始晶格中结晶的，不具有光学声子模。与块体系统相比，在表面，即使是单原子基的金属也可能表现出光学声子，这通常应该是拉曼光谱所能接触到的。此外，作者在最近的一篇论文中指出，利用共振光和涉及表面电子态的光学跃迁，可以大大提高表面声子散射的截面。一种实验和理论相结合的方法现在将阐明潜在的散射机制和在不同金属和表面取向上的共振增强。此外，我们的目标是通过吸附诱导的超结构将研究扩展到表面声子分支的反向折叠的表面吸附。金属原子掺杂剂被用来探索在表面金属合金中局域化光学共振的可能性，从而可能在表面催化中应用。与传统的拉曼散射或表面增强拉曼散射(SERS)不同，表面共振拉曼散射(SRRS)可以用来研究原子平坦的金属表面。共振增强是通过表面电子态发生的，而不是像在SERS中那样通过局域场或等离子激元。因此，SRRS为金属表面的高分辨率振动光谱提供了新的潜力。将在柏林工业大学(TUB)和奥地利林茨的约翰尼斯·开普勒大学(JKU)的两个互补装置中对SRR中的散射机制进行实验研究。相应的原子计算是在密度泛函理论(DFT)的框架下进行的，并在吉森的Justus LieBig大学(JLU)进行了更精细的方法。在最近的一次共同调查中，这三位申请者首次证明了铜(110)表面的拉曼活性。在本方案中，将研究不同金属表面的拉曼活性以及潜在的机制。JKU的实验将集中在裸露和吸附覆盖的铜(110)表面，而其他衬底(如银和金)上的光学波长的依赖关系将在TUB中研究。JLU将提供对所研究的系统及其晶格动力学的原子描述，以阐明系统特定的散射机制。三个相关小组的共同努力使人们能够探索新发现的金属表面性质在表面分析或例如在催化领域的应用的潜力。

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.