The degeneracy is lifted only for certain states obeying the selection rules, in the first order. The first-order splitting in the energy levels for the degenerate states | 2 , 0 , 0 ⟩ ⟩ and | 2 , 1 , 0 ⟩ ⟩ , both corresponding to n = 2, is given by Δ Δ E 2 , 1 , m ℓ ℓ = ± ± | e | ℏ ℏ 2 / ( m e e 2 ) E .