  Using a fully ab-initio methodology, we demonstrate how the lattice
vibrations couple with neutral excitons in monolayer WSe2 and contribute to the
non-radiative excitonic lifetime. We show that only by treating the
electron-electron and electron-phonon interactions at the same time it is
possible to obtain an unprecedented agreement of the zero and
finite-temperature optical gaps and absorption spectra with the experimental
results. The bare energies were calculated by solving the Kohn-Sham equations,
whereas G$_{0}$W$_{0}$ many body perturbation theory was used to extract the
excited state energies. A coupled electron-hole Bethe-Salpeter equation was
solved incorporating the polaronic energies to show that it is the in-plane
torsional acoustic phonon branch that contributes mostly to the A and B exciton
build-up. We find that the three A, B and C excitonic peaks exhibit different
behaviour with temperature, displaying different non-radiative linewidths.
There is no considerable transition in the strength of the excitons with
temperature but A-exciton exhibits darker nature in comparison to C-exciton.
Further, all the excitonic peaks redshifts as temperature rises.
Renormalization of the bare electronic energies by phonon interactions and the
anharmonic lattice thermal expansion causes a decreasing band-gap with
increasing temperature. The zero point energy renormalization (31 meV) is found
to be entirely due to the polaronic interaction with negligible contribution
from lattice anharmonicites. These findings may find a profound impact on
electronic and optoelectronic device technologies based on these monolayers.
