Past exam of the chemistry A course of the University of Cambridge / 2024 / Paper 2 / A10 / e / Solution

The Exciton Rydberg series lies below the unbound electron–hole threshold:Using $E_g=1.240$ eV and $R^{\ast }=0.038241$ eV givesTheir positive exciton binding energies relative to the continuum are $38.24$ meV and $9.56$ meV, respectively.

These are optical electron–hole excitation energies for negligible centre-of-mass momentum, not new occupied one-electron states inside the valence band. The simple model predicts their locations; observing a particular line also requires a nonzero optical transition matrix element and adequate linewidth.