The semiconductor ${\mathrm{ReSe}}_2$ is characterized by a strongly anisotropic optical absorption and is therefore promising as an optically active component in two-dimensional heterostructures. However, the underlying femtosecond dynamics of photoinduced excitations in such materials has not been sufficiently explored. Here, we apply an infrared optical excitation to single-layer ${\mathrm{ReSe}}_2$ grown on a bilayer graphene substrate and monitor the temporal evolution of the excited state signal using time- and angle-resolved photoemission spectroscopy. We measure an optical gap of $(1.53\pm 0.02)\mathrm{eV}$, consistent with resonant excitation of the lowest exciton state. The exciton distribution is tunable via the linear polarization of the pump pulse and exhibits a biexponential decay with time constants given by ${\tau }_1=(110\pm 10)$ fs and ${\tau }_2=(650\pm 70)$ fs, facilitated by interlayer charge transfer to the underlying bilayer graphene and recombination via an in-gap state that is pinned at the Fermi level. By extracting the momentum-resolved exciton distribution we estimate its real-space radial extent to be greater than $(17\pm 1)$ Å, implying significant spatial broadening of the distribution due to screening from the bilayer graphene substrate.

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• Electronic structure
• Excitons
• 2-dimensional systems
• Semiconductors
• Photoemission spectroscopy
• Time & angle resolved photoemission spectroscopy