Exciton Rydberg series ($E_n=E_g-R^{\ast }/n^2$)

The hydrogenic sequence of bound relative electron–hole energies $E_n=E_g-R^{\ast }/n^2$ below the free-pair threshold, where $R^{\ast }=({\mu }^{\ast }/m_e)R_{\mathrm{H}}/{\epsilon }_r^2$. The effective-mass, screened-Coulomb approximation and optical selection rules limit the model.