Investigation of the energetic and dynamic properties of the electronic enhancement mechanism of the surface-enhanced Raman scattering (SERS)

拉曼散射可以通过分子与铸币金属表面的相互作用而显著增强。因此，表面增强拉曼光谱技术在低浓度物质的化学分析等方面具有重要意义。尽管进行了密集的研究，特别是电子(“化学”)SERS机制尚未完全理解，尽管过去曾提出过不同的模型。最值得注意的是，关于化学吸附的分子和金属表面增强拉曼散射底物之间的耦合对分子的能量和动力学性质的影响的详细信息仍然缺乏。在该项目中，时间域的实验将深入了解金属和分子电子态之间相互作用的动态方面，并将这些结果应用于频域SERS测量中的观测。主要集中在金属-吸附耦合对分子内动力学的影响，它直接影响振动模式的激发。为此，将采用不同的泵浦-探测方法(瞬时吸收、时间分辨拉曼散射)以及复杂的非线性技术。结合初始泵浦脉冲和内部时间分辨，瞬时光栅谱(TRG)和相干反斯托克斯拉曼散射(CARS)将有助于跟踪和分析状态和模式选择性驰豫过程。测量将使用ATR(衰减全反射)或Kretschmann配置进行。因此，可以实现更好的可控条件。在其他方面，将调查主要假设的SERS电子增强机制与电磁增强机制的明确分离是否真的有意义。进一步研究了相干非线性光谱表面增强的效率。例如，当分子被吸附到SERS衬底上时，超拉曼散射经历了显著的增强，但CARS信号的增强远远低于理论预测。

Raman scattering can be drastically enhanced by the interaction of the molecules with coinage metal surfaces. Therefore, SERS (Surface Enhanced Raman Spectroscopy) is of great interest e.g. for the chemical analytics of low-concentrated substances. In spite of intense research, specifically the electronic ("chemical") SERS mechanism has not been completely understood, even though different models have been suggested in the past. Most notably, detailed information about the influence of the coupling between chemisorbed molecules and metallic SERS substrates on the energetic and dynamic properties of the molecules is still missing. In the project, experiments in the time domain will give insights into dynamic aspects of the interaction between the electronic states of metal and molecules and will implicate these results in observations made in frequency domain SERS measurements. The main focus will be on the effect of the metal-adsorbate coupling on the intramolecular dynamics, which directly influence the excitation of vibrational modes. For this purpose, different pump-probe methods (transient absorption, time-resolved Raman scattering) as well as complex nonlinear techniques will be applied. In combination with an initial pump pulse as well as internally time-resolved, transient grating spectroscopy (TRG) and coherent anti-Stokes Raman scattering (CARS) will help to trace and analyze state and mode-selectively relaxation processes. Measurements will be performed using the ATR (attenuated total reflection) or Kretschmann configuration. Thereby, much better controllable conditions can be realized. Amongst other things, it will be investigated whether the mostly assumed clear separation of the electronic enhancement mechanism of SERS from the electromagnetic one really makes sense. A further point is the investigation of the efficiency of the surface enhancement of coherent nonlinear spectroscopy. While e.g. hyper Raman scattering experiences striking enhancement when molecules are adsorbed to SERS substrates, the enhancement of CARS signals falls far short of theoretical predictions.