  The coupling of excitons with atomic vibrations plays a pivotal role on the
nonequilibrium optical properties of layered semiconductors. However,
addressing the dynamical interaction between excitons and phonons represents a
hard task both experimentally and theoretically. By means of time-resolved
broadband optical reflectivity combined with state-of-the-art ab-initio
calculations of a bismuth triiodide single crystal, we unravel the universal
spectral fingerprints of exciton--phonon coupling in layered semiconductors.
Furthermore, we microscopically relate a photoinduced coherent energy
modulation of the excitonic resonance to coherent optical phonons, thereby
tracking the extent of the photoinduced atomic displacement in real-space. Our
findings represent a step forward on the road to coherent manipulation of the
excitonic properties on ultrafast timescales.
