Heterostructures of layered transition metal dichalcogenides (TMDs) host long-lived, tunable excitons, making them intriguing candidates for material-based quantum information applications. Light absorption in these systems induces a plethora of optically excited states that hybridize both interlayer and intralayer characteristics, providing a distinctive starting point for their relaxation processes, in which the interplay between generated electron-hole pairs and their scattering with phonons play a key role. We present a first-principles theoretical approach to compute phonon-induced exciton decomposition due to rapid occupation of electron-hole pairs with finite momentum and opposite spin. Using the ${\mathrm{MoSe}}_2/{\mathrm{WSe}}_2$ heterostructure as a case study, we observe a reduction in the optical activity of bright states upon phonon scattering already in the first few femtoseconds after a photoexcitation, driving exciton interlayer delocalization and subsequent variations in the exciton spin. Our results reveal an unexpected and previously unexplored starting point for exciton relaxation dynamics, suggesting increased availability for coherent interactions and nonradiative processes through ultrafast changes in exciton momentum, spatial, and spin properties upon light excitation.

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• Excitons
• First-principles calculations
• Layered semiconductors
• Transition metal dichalcogenides
• Band structure methods
• Bethe-Salpeter equation
• Density matrix methods
• GW method