Each of the exciton resonances can produce a peak to the absorption spectrum when the photon energy matches with E λ λ . For direct-gap semiconductors, the oscillator strength is proportional to the product of dipole-matrix element squared and | ∑ ∑ k ϕ ϕ λ λ ( k ) | 2 that vanishes for all states except for those that are spherically symmetric. In other words, F λ λ is nonvanishing only for the s -like states, following the quantum-number convention of the hydrogen problem. Therefore, optical spectrum of direct-gap semiconductors produces an absorption resonance only for the s -like state. The width of the resonance is determined by the corresponding dephasing constant.