Unveiling anisotropic exciton splitting and momentum-dark states in ReS₂

ReS₂ has a distorted 1T crystal structure, exhibiting non-equilibrium excitonic properties in electrically decoupled van der Waals layers. Our study finds four excitonic emissions which are highly polarized with dipole moments oriented along different directions with respect to the crystal axis. Dipole orientations of two newly discovered excitons X₃ and X₄ are aligned along two strong previously known excitons X₁ and X₂ respectively. In addition, we observe an anomalous variation in the photoluminescence intensity near 30K which is the signature of momentum-forbidden dark states below the excitonic resonances, as indicated by our rate equation model. Our work shed light on the quasi-direct nature of the electronic band structure for ReS₂ multilayer system.