This study experimentally determines the exciton binding energy and the nonhydrogenic Rydberg series in monolayer WS₂, a two-dimensional semiconductor. The researchers measured the energies of the ground and first four excited excitonic states, finding a large exciton binding energy of 0.32 eV and a deviation from the hydrogenic Rydberg series. These results are explained by a microscopic theory where the non-local nature of the effective dielectric screening modifies the Coulomb interaction. The strong but unconventional electron-hole interactions are expected to be common in atomically thin materials. Monolayer transition metal dichalcogenides (TMDs), such as WS₂, exhibit unique physical properties due to their reduced dimensionality. These materials are promising for fundamental studies and applications in optoelectronics and valleytronics due to their direct bandgap. Recent advances include enhanced photoluminescence, efficient spin-valley coupling, pronounced many-body effects, and high-performance in field-effect transistors. The 2D nature of TMDs enhances the Coulomb interaction, leading to the formation of bound electron-hole pairs (excitons) that dominate optical and charge-transport properties. Understanding exciton formation is crucial for both fundamental physics and device applications. Theoretical predictions suggest exciton binding energies up to 1 eV, but direct measurements in TMDs are still lacking. The study experimentally and theoretically investigates exciton properties in mono- and few-layer TMDs, identifying and characterizing the ground-state exciton and the full sequence of excited states. Analyzing optical reflection spectra, the researchers estimate the 1s exciton binding energy as 0.32 eV and the quasiparticle gap as 2.41 eV. The results show significant deviations from the conventional hydrogenic model, explained by a microscopic theory highlighting the unique electron-hole interaction in TMDs. The study focuses on WS₂, a representative TMD with a large spin-orbit splitting between A and B excitons, allowing for the study of low-energy excitons. The electronic transitions in WS₂ samples exhibit narrow spectral features, enabling the identification of many excited excitonic states. The study also monitors the spectral position of the 2s resonance for varying thickness of WS₂ samples, showing a strong increase in both the exciton binding energy and the quasi-particle band gap with decreasing layer thickness. The large binding energy of 0.32 eV and the non-hydrogenic behavior of the intra-excitonic states in monolayer WS₂ are expected to be common in other TMD materials. These properties suggest high thermal stability of excitons and potential applications in photonic and excitonic devices. The non-hydrogenic series of excited states allows for efficient exploitation of intra-excitonic processes in the far-IR and THz spectral range. The study also highlights the potential of WS₂ as a link between inorganic semThis study experimentally determines the exciton binding energy and the nonhydrogenic Rydberg series in monolayer WS₂, a two-dimensional semiconductor. The researchers measured the energies of the ground and first four excited excitonic states, finding a large exciton binding energy of 0.32 eV and a deviation from the hydrogenic Rydberg series. These results are explained by a microscopic theory where the non-local nature of the effective dielectric screening modifies the Coulomb interaction. The strong but unconventional electron-hole interactions are expected to be common in atomically thin materials. Monolayer transition metal dichalcogenides (TMDs), such as WS₂, exhibit unique physical properties due to their reduced dimensionality. These materials are promising for fundamental studies and applications in optoelectronics and valleytronics due to their direct bandgap. Recent advances include enhanced photoluminescence, efficient spin-valley coupling, pronounced many-body effects, and high-performance in field-effect transistors. The 2D nature of TMDs enhances the Coulomb interaction, leading to the formation of bound electron-hole pairs (excitons) that dominate optical and charge-transport properties. Understanding exciton formation is crucial for both fundamental physics and device applications. Theoretical predictions suggest exciton binding energies up to 1 eV, but direct measurements in TMDs are still lacking. The study experimentally and theoretically investigates exciton properties in mono- and few-layer TMDs, identifying and characterizing the ground-state exciton and the full sequence of excited states. Analyzing optical reflection spectra, the researchers estimate the 1s exciton binding energy as 0.32 eV and the quasiparticle gap as 2.41 eV. The results show significant deviations from the conventional hydrogenic model, explained by a microscopic theory highlighting the unique electron-hole interaction in TMDs. The study focuses on WS₂, a representative TMD with a large spin-orbit splitting between A and B excitons, allowing for the study of low-energy excitons. The electronic transitions in WS₂ samples exhibit narrow spectral features, enabling the identification of many excited excitonic states. The study also monitors the spectral position of the 2s resonance for varying thickness of WS₂ samples, showing a strong increase in both the exciton binding energy and the quasi-particle band gap with decreasing layer thickness. The large binding energy of 0.32 eV and the non-hydrogenic behavior of the intra-excitonic states in monolayer WS₂ are expected to be common in other TMD materials. These properties suggest high thermal stability of excitons and potential applications in photonic and excitonic devices. The non-hydrogenic series of excited states allows for efficient exploitation of intra-excitonic processes in the far-IR and THz spectral range. The study also highlights the potential of WS₂ as a link between inorganic sem