  We report many-body perturbation theory calculations of excited-state
properties of distorted 1-T diamond-chain monolayer rhenium disulfide (ReS2)
and diselenide (ReSe2). Electronic self-energy substantially enhances their
quasiparticle band gaps and, surprisingly, converts monolayer ReSe2 to a
direct-gap semiconductor, which was, however, regarded to be an indirect one by
density-functional-theory calculations. Their optical absorption spectra are
dictated by strongly bound excitons. Unlike hexagonal structures, the
lowest-energy bright exciton of distorted 1-T ReS2 exhibits a perfect figure-8
shape polarization dependence but those of ReSe2 only exhibit a partial
polarization dependence, which results from two nearly-degenerated bright
excitons whose polarization preferences are not aligned. Our first-principles
calculations are in agreement with experiments and pave the way for
optoelectronic applications.
