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The optical properties of MoS2 monolayers are dominated by excitons, but forspectrally broad optical transitions in monolayers exfoliated directly ontoSiO2 substrates detailed information on excited exciton states is inaccessible.Encapsulation in hexagonal boron nitride (hBN) allows approaching thehomogenous exciton linewidth, but interferences in the van der Waalsheterostructures make direct comparison between transitions in optical spectrawith different oscillator strength more challenging. Here we reveal inreflectivity and in photoluminescence excitation spectroscopy the presence ofexcited states of the A-exciton in MoS2 monolayers encapsulated in hBN layersof calibrated thickness, allowing to extrapolate an exciton binding energy ofabout 220 meV. We theoretically reproduce the energy separations and oscillatorstrengths measured in reflectivity by combining the exciton resonancescalculated for a screened two-dimensional Coulomb potential with transfermatrix calculations of the reflectivity for the van der Waals structure. Ouranalysis shows a very different evolution of the exciton oscillator strengthwith principal quantum number for the screened Coulomb potential as compared tothe ideal two-dimensional hydrogen model.

Optical spectroscopy of excited exciton states in MoS2 monolayers in van der Waals heterostructures