  Exciton dynamics dictate the evolution of photoexcited carriers in
photovoltaic and optoelectronic devices. However, interpreting their
experimental signatures is a challenging theoretical problem due to the
presence of both electron-phonon and many-electron interactions. We develop and
apply here a first-principles approach to exciton dynamics resulting from
exciton-phonon coupling in monolayer MoS2 and reveal the highly selective
nature of exciton-phonon coupling due to the internal spin structure of
excitons, which leads to a surprisingly long lifetime of the lowest energy
bright A exciton. Moreover, we show that optical absorption processes
rigorously require a second-order perturbation theory approach, with photon and
phonon treated on an equal footing, as proposed by Toyozawa and Hopfield. Such
a treatment, thus far neglected in first-principles studies, gives rise to
off-diagonal exciton-phonon coupling matrix elements, which are critical for
the description of dephasing mechanisms, and yields exciton linewidths in
excellent agreement with experiment.
