摘要： Two-dimensional Transition Metal Dichalcogenides (TMDs) have been widely studied because of the peculiar electronic band structure and the strong excitonic effects. In these materials the large spin-orbit coupling lifts the spin degeneracy of the valence (VB) and the conduction band (CB) giving rise to the A and B interband excitonic transitions. In monolayer WS2, the spins of electrons in the lowest CB and in the highest VB at K/K’ point of the Brillouin zone are antiparallel resulting in an intravalley dark exciton state at a lower energy than the bright exciton. Here we use two-colour helicity-resolved pump-probe spectroscopy to directly resolve the intravalley spin-?ip process of the photoexcited electrons in the CB of single-layer WS2. We ?nd that at T=77 K this process occurs on a sub-ps time scale and it is signi?cantly dependent on the temperature, strongly pointing to a phonon assisted relaxation process. Our results shed light on the intravalley spin relaxation process in single-layer WS2, determining the formation of the intravalley dark exciton, which we measure to occur on a sub-ps timescale. The study of dark excitons formation dynamics is important for designing TMD-based electronic/photonic devices.