External screening and lifetime of exciton population in single-layer ReSe2 probed by time- and angle-resolved photoemission spectroscopy

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 $(1.53\ifmmode\pm\else\textpm\fi{}0.02)\phantom{\rule{0.28em}{0ex}}\mathrm{eV}$, 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 ${\ensuremath{\tau}}₁=(110\ifmmode\pm\else\textpm\fi{}10)$ fs and ${\ensuremath{\tau}}₂=(650\ifmmode\pm\else\textpm\fi{}70)$ 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 $(17\ifmmode\pm\else\textpm\fi{}1)$ \AA{}, implying significant spatial broadening of the distribution due to screening from the bilayer graphene substrate.