Direct Determination of Band-Gap Renormalization in the Photoexcited Monolayer <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi>MoS</mml:mi></mml:mrow><mml:mn>2</mml:mn></mml:msub></mml:mrow></mml:math>

A key feature of monolayer semiconductors, such as transition-metal dichalcogenides, is the poorly screened Coulomb potential, which leads to a large exciton binding energy (E_{b}) and strong renormalization of the quasiparticle band gap (E_{g}) by carriers. The latter has been difficult to determine due to a cancellation in changes of E_{b} and E_{g}, resulting in little change in optical transition energy at different carrier densities. Here, we quantify band-gap renormalization in macroscopic single crystal MoS_{2} monolayers on SiO_{2} using time and angle-resolved photoemission spectroscopy. At an excitation density above the Mott threshold, E_{g} decreases by as much as 360 meV. We compare the carrier density-dependent E_{g} with previous theoretical calculations and show the necessity of knowing both doping and excitation densities in quantifying the band gap.