Evaluating the contribution of electromagnetic nearfield gradients in TERS

Tip-enhanced Raman spectroscopy (TERS) extends the spatial resolution of conventional Raman far beyond the diffraction limit by merging the advantages of both surface-enhanced Raman spectroscopy (SERS) and scanning probe microscopy (SPM). While most theoretical models focus on either electromagnetic (EM) enhancement or chemical (CM) effects - and typically consider only the component of the polarizability tensor normal to the substrate (αzz) - we present a fully quantum-mechanical approach that accounts for all components of the polarizability tensor, allowing to evaluate the impact of spatial inhomogeneity in the electromagnetic nearfield on the Raman response. These may become particularly pronounced for Raman modes which are, for symmetry reasons, dark in the field enhancement direction normal to the substrate (here the z-direction). By scanning a single metal atom mimicking a plasmonic tip at various positions above a flat planar sample molecule, we demonstrate how the tip-molecule interactions shift the electronic transitions and thereby modify the resonance Raman response. In particular, some off-diagonal components (αxy, αyx) become crucial for activating certain vibrational modes that are absent for the isolated molecule. Decreasing the tip-sample distance reveals how the z-directed charge-transfer states enhance TERS signals out of the molecular plane. Overall, our findings underscore the need to include different components of the polarizability tensor and to account for both EM and CM contributions for an accurate description of TERS spectra.