A dangling bond adds an extra energy level between the valence band and conduction band of a lattice. This allows for absorption and emission at longer wavelengths, because electrons can take smaller energy steps by moving to and from this extra level. The energy of the photons absorbed by or emitted from this level is not exactly equal to the energy difference between the bottom of the conduction band and the dangling bond or the top of the valence band and the dangling bond. This is due to lattice relaxation which causes a Franck-Condon shift in the energy. This shift accounts for the difference between a tight-binding calculation of these energy differences and the experimentally measured energies.