  The excitonic spectra of single layer GeS and GeSe are predicted by ab initio
GW-Bethe Salpeter equation calculations. G 0 W 0 calculations for the band
structures find a fundamental band gap of 2.85 eV for GeS and 1.70 eV for GeSe
monolayer. However, excitons are tightly bound, specially in GeS at the
{\Gamma} point, where the quasi-particle interactions are so strong that they
shift the {\Gamma} exciton peak energy into the visible range and below the
off-{\Gamma} exciton peak. The lowest energy excitons in both materials are
excited by light along the zigzag direction and have exciton binding energies
of 1.05 eV and 0.4 eV, respectively, but despite the strong binding, the
calculated binding energies are in agreement with a Mott-Wannier model.
