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

The semiconductor ReSe2 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 ReSe2 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 +/- 0.02) 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 v1 = (110 +/- 10) fs and v2 = (650 +/- 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 +/- 1) angstrom, implying significant spatial broadening of the distribution due to screening from the bilayer graphene substrate.