bilayers*

Exciton dynamics have been investigated extensively in transition metal dichalcogenide (TMDC) monolayers, but comparatively little is known of their behavior in bilayers, due to the presence of multiple low-energy valleys in bilayer TMDC band structures. We find that bright exciton decoherence occurs on a much faster time scale in MoSe2 bilayers than in monolayers, and is limited by pure dephasing processes. Remarkable agreements between the measured and calculated homogeneous linewidths in both the monolayer and bilayer are found by taking into account all relevant exciton-phonon scattering processes. Our microscopic model identies that phonon-emission processes facilitate scattering events from the K valley to lower energy Γ and Λ valleys in the bilayer and induce pure dephasing.