  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 MoSe$_2$/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 proceeding
the 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 non-radiative processes through
ultrafast changes in exciton momentum, spatial, and spin properties upon light
excitation.
