Theoretical analysis of optical excitation and luminescence in MoS$_2$ monolayers

We analyze the absorption and circularly polarized luminescence spectra of MoS$_2$ monolayers. We show that indirect optical transitions can fully explain the observed decrease in circular polarization degree when increasing the lattice temperature or the exciting photon energy. This spin-conserving optical process is assisted by electron-phonon or electron-impurity interactions giving rise to intervalley transitions to intermediate virtual states. Spin-flip mechanisms, on the other hand, are shown to be insufficient in explaining the experimental results due to their relatively long timescales compared with the radiative timescales in monolayer dichalcogenides (tens of ps).