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

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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.