  Twisted bilayer two-dimensional transition-metal dichalgocenides (TMDCs)
exhibit a range of novel phenomena such as the formation of moir\'e excitons
and strongly correlated phases. The coupling between the layers is crucial for
the resulting physical properties and depends not just on the twist angle but
also on the details of the fabrication process. Here, we present an approach
for the analysis of this interlayer coupling via Raman spectroscopy. By
exciting the C-exciton resonance of the TMDCs, optical (high-frequency)
interlayer Raman modes are activated that are too weak in intensity at
excitation energies far below the C exciton. This is due to the wave function
of the C exciton, which expands - in contrast to A and B excitons - over both
layers and, therefore, couples the layers electronically. We present optical
interlayer Raman modes in twisted 2L TMDCs and show that they can be used as a
measure of the interlayer coupling between the individual layers.
