  Inversion symmetry breaking and three-fold rotation symmetry grant the valley
degree of freedom to the robust exciton in monolayer transition metal
dichalcogenides (TMDCs), which can be exploited for valleytronics applications.
However, the short lifetime of the exciton significantly constrains the
possible applications. In contrast, dark exciton could be long-lived but does
not necessarily possess the valley degree of freedom. In this work, we report
the identification of the momentum-dark, intervalley exciton in monolayer WSe2
through low-temperature magneto-photoluminescence (PL) spectra. Interestingly,
the intervalley exciton is brightened through the emission of a chiral phonon
at the corners of the Brillouin zone (K point), and the pseudoangular momentum
(PAM) of the phonon is transferred to the emitted photon to preserve the valley
information. The chiral phonon energy is determined to be ~ 23 meV, based on
the experimentally extracted exchange interaction (~ 7 meV), in excellent
agreement with the theoretical expectation of 24.6 meV. The long-lived
intervalley exciton with valley degree of freedom adds an exciting
quasiparticle for valleytronics, and the coupling between the chiral phonon and
intervalley exciton furnishes a venue for valley spin manipulation.
