The study experimentally determines the ground and first four excited excitonic states of monolayer WS₂, a model system for atomically thin two-dimensional (2D) semiconductor crystals. The researchers find a large exciton binding energy of 0.32 eV and deviations from the usual hydrogenic Rydberg series of energy levels. These results are explained using a microscopic theory that accounts for the non-local nature of effective dielectric screening, which modifies the functional form of the Coulomb interaction. This strong but unconventional electron-hole interaction is expected to be common in atomically thin materials. The study also investigates the influence of material thickness on exciton properties, showing that the 2s resonance shifts to higher energies as the layer thickness decreases, while the 1s resonance remains relatively unchanged. These findings highlight the potential of monolayer WS₂ for optoelectronic devices due to its high thermal stability and high oscillator strength.The study experimentally determines the ground and first four excited excitonic states of monolayer WS₂, a model system for atomically thin two-dimensional (2D) semiconductor crystals. The researchers find a large exciton binding energy of 0.32 eV and deviations from the usual hydrogenic Rydberg series of energy levels. These results are explained using a microscopic theory that accounts for the non-local nature of effective dielectric screening, which modifies the functional form of the Coulomb interaction. This strong but unconventional electron-hole interaction is expected to be common in atomically thin materials. The study also investigates the influence of material thickness on exciton properties, showing that the 2s resonance shifts to higher energies as the layer thickness decreases, while the 1s resonance remains relatively unchanged. These findings highlight the potential of monolayer WS₂ for optoelectronic devices due to its high thermal stability and high oscillator strength.