Light
interaction with metal nanostructures exposes exciting phenomena
such as strong amplification and localization of electromagnetic fields.
In surface-enhanced Raman spectroscopy (SERS), the strong signal amplification
is attributed to two fundamental mechanisms, electromagnetic and chemical
enhancement (EM and CM, respectively). While the EM mechanism is accepted
as the main responsible for signal amplification, a long-standing
controversy on the CM mechanism’s role still prevails. The
CM contribution can be evidenced when compared to the nonenhanced
(or bulk) Raman signal as a change in intensity ratios, peak shifts,
or appearance of new Raman modes. However, it is also possible to
induce similar spectral variations by changing the relative orientation
between the electric field and molecule or when a high electric field
gradient is achieved. Therefore, in this work, we show specific spectral
changes in SERS affected by the molecular orientation, while changes
in other modes can be attributed to chemical enhancement. On the basis
of our experimental and quantum chemical results for cobalt phthalocyanine,
we identify low-frequency Raman modes (LFMs) sensitive to charge-transfer
compared to high-frequency modes (HFMs) that are rather sensitive
to geometrical effects and temperature changes. These results provide
new evidence on the role of molecule excitation/polarization that
comes now as a more general and dominant effect than the chemical
enhancement mechanism so far attributed to charge-transfer processes.
These findings make it possible to engineer multifunctional Raman
molecular probes with selective sensitivity to the local environment
(HFMs) and charge-transfer processes (LFMs).