  Many properties of layered materials change as they are thinned from their
bulk forms down to single layers, with examples including indirect-to-direct
band gap transition in 2H semiconducting transition metal dichalcogenides as
well as thickness-dependent changes in the valence band structure in
post-transition metal monochalcogenides and black phosphorus. Here, we use
angle-resolved photoemission spectroscopy to study the electronic band
structure of monolayer ReSe$_{2}$, a semiconductor with a distorted 1T
structure and in-plane anisotropy. By changing the polarization of incoming
photons, we demonstrate that for ReSe$_{2}$, in contrast to the 2H materials,
the out-of-plane transition metal $d_{z^{2}}$ and chalcogen $p_{z}$ orbitals do
not contribute significantly to the top of the valence band which explains the
reported weak changes in the electronic structure of this compound as a
function of layer number. We estimate a band gap of 1.7 eV in pristine
ReSe$_{2}$ using scanning tunneling spectroscopy and explore the implications
on the gap following surface-doping with potassium. A lower bound of 1.4 eV is
estimated for the gap in the fully doped case, suggesting that doping-dependent
many-body effects significantly affect the electronic properties of ReSe$_{2}$.
Our results, supported by density functional theory calculations, provide
insight into the mechanisms behind polarization-dependent optical properties of
rhenium dichalcogenides and highlight their place amongst two-dimensional
crystals.
