  We present a transient absorption setup combining broadband detection over
the visible-UV range with high temporal resolution ($\sim$20fs) which is
ideally suited to trigger and detect vibrational coherences in different
classes of materials. We generate and detect coherent phonons (CPs) in single
layer (1L) MoS$_2$, as a representative semiconducting 1L-transition metal
dichalcogenide (TMD), where the confined dynamical interaction between excitons
and phonons is unexplored. The coherent oscillatory motion of the out-of-plane
$A'_{1}$ phonons, triggered by the ultrashort laser pulses, dynamically
modulates the excitonic resonances on a timescale of few tens fs. We observe an
enhancement by almost two orders of magnitude of the CP amplitude when detected
in resonance with the C exciton peak, combined with a resonant enhancement of
CP generation efficiency. Ab initio calculations of the change in 1L-MoS$_2$
band structure induced by the $A'_{1}$ phonon displacement confirm a strong
coupling with the C exciton. The resonant behavior of the CP amplitude follows
the same spectral profile of the calculated Raman susceptibility tensor. This
demonstrates that CP excitation in 1L-MoS$_2$ can be described as a Raman-like
scattering process. These results explain the CP generation process in 1L-TMDs,
paving the way for coherent all-optical control of excitons in layered
materials in the THz frequency range.
