Nanoscopy

Surface-enhanced Raman spectra are simulated using a combined classical electrodynamics / real-time time-dependent density functional theory approach and compared to experiments. Emphasis is put on discerning between chemical and electromagnetic enhancement. Therefore, three different calculation scenarios are investigated using emph{para}-Nitrothiophenol as a test molecule. In the first one, corrsponding to electromagnetic enhancement, we simulate the molecule alone with ab initio computations incorporating the electromagnetic field emitted by a nanoparticle. Chemical enhancement is modeled in the second scenario, where we include not only the molecule into the quantum chemistry calculations but also metal atoms of the nanoparticle. Here, the field from classical electrodynamics is not considered. In the third scenario, the former two setups are combined and demanding simulations of the molecule, metal atoms, and electromagnetic field are compared to our experimental results. Based on our analysis well beyond the state-of-the-art that usually considers only one of the first two scenarios described above, we show here on rigorous grounds that the electromagnetic effect leads to increased absolute Raman scattering cross sections but no change of the relative intensities. In contrast, the chemical effect leads to changes in relative peak height and also to newly emerging bands in the spectrum. These findings will have major implications in any study that concerns the interaction of molecules with metallic nanostructures.