  The monolayer transition metal dichalcogenides are an emergent semiconductor
platform exhibiting rich excitonic physics with coupled spin-valley degree of
freedom and optical addressability. Here, we report a new series of low energy
excitonic emission lines in the photoluminescence spectrum of ultraclean
monolayer WSe2. These excitonic satellites are composed of three major peaks
with energy separations matching known phonons, and appear only with electron
doping. They possess homogenous spatial and spectral distribution, strong power
saturation, and anomalously long population (> 6 ${\mu}$s) and polarization
lifetimes (> 100 ns). Resonant excitation of the free inter- and intra-valley
bright trions leads to opposite optical orientation of the satellites, while
excitation of the free dark trion resonance suppresses the satellites
photoluminescence. Defect-controlled crystal synthesis and scanning tunneling
microscopy measurements provide corroboration that these features are dark
excitons bound to dilute donors, along with associated phonon replicas. Our
work opens opportunities to engineer homogenous single emitters and explore
collective quantum optical phenomena using intrinsic donor-bound excitons in
ultraclean 2D semiconductors.

Mesoscale and Nanoscale Physics;Materials Science