Excited states of excitons in MoSe$_2$ and WSe$_2$ monolayers

Excitons in MoSe$_2$ and WSe$_2$ monolayers encapsulated with hexagonal boron nitride have been studied using optical reflectance spectroscopy. The ground and excited states of A- and B-excitons have been studied at temperatures from liquid helium to room temperature. The lines of excitons A:$1s$ , B:$1s$ and their excited states A:$2s$ , A:$3s$ , and B:$2s$ are clearly observed in the reflectance spectrum. The observed line shapes of the reflection spectrum of transition metal dichalcogenide monolayers depend on the thickness of the hexagonal boron nitride layers used in the structure and are in good agreement with the numerical simulation using the transfer matrix method. For the first time, the values of the reduced masses of B-excitons have been obtained from experimental data and the performed calculations of the exciton binding energy.