  We present an analytical investigation of the optical absorption spectrum of
monolayer molybdenumdisulfide. Based on the density matrix formalism, our
approach gives insights into the microscopic origin of excitonic transitions,
their relative oscillator strength, and binding energy. We show analytical
expressions for the carrier-light coupling element, which contains the optical
selection rules and well describes the valleyselective polarization in MoS2. In
agreement with experimental results, we find the formation of strongly bound
electron-hole pairs due to the efficient Coulomb interaction. The absorption
spectrum of MoS2 on a silicon substrate features two pronounced peaks at 1.91
eV and 2.05 eV corresponding to the A and B exciton, which are characterized by
binding energies of 420 meV and 440 meV, respectively. Our calculations reveal
their relative oscillator strength and predict the appearance of further
low-intensity excitonic transitions at higher energies. The presented approach
is applicable to other transition metal dichalcogenides and can be extended to
investigations of trion and biexcitonic effects.
