Ab initio Hartree—Fock calculations in the single-configuration approximation predict that the 3s 3p^{6 2}S_½ level of Cl _I lies far above the ionization limit, although this level is observed to be the lowest of all even J = 1/2 levels other than 3s² 3p⁴ 4s⁴P_½, ²P_½. The solution of this anomaly lies in configuration interaction with the high-lying portion of the 3s² 3p⁴ ∊d²S_½ continuum, which is sufficiently strong to depress the computed s p⁶ level to its observed position. This interaction with the continuum is an extension of the well-known interaction of sp^{ω + 1} with the discrete s²p^{ω - 1}nd Rydberg series, and is important in all neutral third-row elements Al to Cl, as well as in Br and I and probably other fourth- and fifth-row elements. The effect is, however, somewhat smaller in Br ₁ and I ₁ than in Cl ₁, with the result that there exists no bromine nor iodine level that is primarily sp⁶ in nature. The interactions produce large changes in computed transition probabilities and photoionization cross sections.

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