  The weakly screened electron-hole interactions in an atomically thin
semiconductor not only downshift its excitation spectrum from a quasiparticle
one, but also redistribute excitation energies and wavefunction characters with
profound effects on diverse modes of material response, including the
exciton-phonon scattering processes accessible to resonant Raman measurements.
Here we develop a first-principles framework to calculate frequency-dependent
resonant Raman intensities that includes excitonic effects and goes beyond the
Placzek approximation. We show how excitonic effects in MoS2 strongly regulate
Raman scattering amplitudes and thereby explain the puzzling near-absence of
resonant Raman response around the A and B excitons (which produce very strong
signals in optical absorption), and also the pronounced strength of the
resonant Raman response from the C exciton. Furthermore, this efficient
perturbative approach reduces the number of GW- BSE calculations from two per
Raman mode (in finite displacement) to one for all modes and affords natural
extension to higher-order resonant Raman processes.
