We demonstrate that, in monolayers (MLs) of semiconducting transition metal dichalcogenides, the s-type Rydberg series of excitonic states follows a simple energy ladder: ǫn = −Ry * /(n + δ) 2 , n=1,2,. . . , in which Ry * is very close to the Rydberg energy scaled by the dielectric constant of the medium surrounding the ML and by the reduced effective electron-hole mass, whereas the ML polarizability is only accounted for by δ. This is justified by the analysis of experimental data on excitonic resonances, as extracted from magneto-optical measurements of a high-quality WSe2 ML encapsulated in hexagonal boron nitride (hBN), and well reproduced with an analytically solvable Schrödinger equation when approximating the electron-hole potential in the form of a modified Kratzer potential. Applying our convention to other, MoSe2, WS2, MoS2 MLs encapsulated in hBN, we estimate an apparent magnitude of δ for each of the studied structures. Intriguingly, δ is found to be close to zero for WSe2 as well as for MoS2 monolayers, what implies that the energy ladder of excitonic states in these two-dimensional structures resembles that of Rydberg states of a three-dimensional hydrogen atom.

Reflectance and magneto-reflectance experiments together with theoretical modelling based on the k · p approach have been employed to study the evolution of direct bandgap excitons in MoS2 layers with a thickness ranging from mono-to trilayer. The extra excitonic resonances observed in MoS2 multilayers emerge as a result of the hybridization of Bloch states of each sub-layer due to the interlayer coupling. The properties of such excitons in bi-and trilayers are classified by the symmetry of corresponding crystals. The inter-and intralayer character of the reported excitonic resonances is fingerprinted with the magneto-optical measurements: the excitonic g-factors of opposite sign and of different amplitude are revealed for these two types of resonances. The parameters describing the strength of the spin-orbit interaction are estimated for bi-and trilayer MoS2.