While valleys (energy extrema) are present in all band structures of solids, their preeminent role in determining exciton resonances and dynamics in atomically thin transition metal dichalcogenides (TMDC) is unique. Using two-dimensional coherent electronic spectroscopy, we find that exciton decoherence occurs on a much faster timescale in ${\mathrm{MoSe}}_2$ bilayers than that in the monolayers. We further identify two population relaxation channels in the bilayer, a coherent and an incoherent one. Our microscopic model reveals that phonon-emission processes facilitate scattering events from the $K$ valley to other lower-energy $\mathrm{\Gamma }$ and $\mathrm{\Lambda }$ valleys in the bilayer. Our combined experimental and theoretical studies unequivocally establish different microscopic mechanisms that determine exciton quantum dynamics in TMDC monolayers and bilayers. Understanding exciton quantum dynamics provides critical guidance to the manipulation of spin-valley degrees of freedom in TMDC bilayers.

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• Excitons
• Phonons
• Quantum coherence & coherence measures
• Quantum wells
• Semiconductor compounds
• Transition metal dichalcogenides
• Femtosecond laser spectroscopy
• Four-wave mixing
• Photoluminescence