Quantum chemical investigation towards Raman enhancement effects
A well-known drawback of Raman spectroscopy is the low signal intensity. Therefore, the present thesis aims to elucidate the fundamental principles in state-of-the-art signalenhanced Raman spectroscopy, i.e. resonance Raman (RR), surface-enhanced Raman scattering (SERS) and tip-enhanced Raman scattering (TERS). Firstly, a new approach to evaluate RR scattering is discussed. This new approach, called real-time method, can evaluate the polarizabilitiy tensor in the time domain. The new method is benchmarked by being compared to other two well-known methods. All three methods give comparable results, and consequently, the novel computational approach is validated. Afterwards, the chemical e_ect of SERS is studied by means of a theoretical spectroscopic analysis of the cis/trans-isomers of a molecular switch, the penta-2,4-dienoic acid, attached to gold clusters of di_erent size (1, 2 and 20 gold atoms). By simulation of vibrational (IR, Raman and RR) and electronic spectroscopy (UV-vis), the e_ect of the molecule-metal nanoparticle interaction can be studied. Special emphasis is put on the resonance Raman spectra for the study of the isomers. In the present case, resonance Raman scattering is best suited to discriminate between the isomers on the gold clusters. Finally a full quantum chemical description of the non-resonant chemical e_ects of the Raman spectrum of an adenine molecule mapped by a silver tip is presented. The tip is modeled as a single silver atom and as a small silver cluster. Pronounced changes in the Raman pattern and its intensities depending on the conformation of the nanoparticlesubstrate system are found, concluding that the spatial resolution of the chemical contribution in TERS can be in the sub-nm range.
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