  Lately, experimental evidence of unexpectedly extremely high spatial
resolution of tip-enhanced Raman scattering (TERS) has been demonstrated.
Theoretically, two different contributions are discussed: an electromagnetic
effect, leading to a spatially confined near field due to plasmonic
excitations; and the so-called chemical effect originating from the locally
modified electronic structure of the molecule due to the close proximity of the
plasmonic system. Most of the theoretical efforts have concentrated on the
electromagnetic contribution or the chemical effect in case of non-resonant
excitation. In this work, we present a fully quantum mechanical description
including non-resonant and resonant chemical contributions as well as
charge-transfer phenomena of these molecular-plasmonic hybrid system at the
density functional and the time-dependent density functional level of theory.
We consider a surface-immobilized tin(II) phthalocyanine molecule as the
molecular system, which is minutely scanned by a plasmonic tip, modeled by a
single silver atom. These different relative positions of the Ag atom to the
molecule lead to pronounced alterations of the Raman spectra. These Raman
spectra vary substantially, both in peak positions and several orders of
magnitude in the intensity patterns under non-resonant and resonant conditions,
and also, depending on, which electronic states are addressed. Our
computational approach reveals that unique - non-resonant and resonant -
chemical interactions among the tip and the molecule significantly alter the
TERS spectra and are mainly responsible for the high, possibly sub-Angstrom
spatial resolution.
