Rydberg states of the HeH⁺ ion calculated by <i>ab initio</i> methods: potential energies and effective principal quantum numbers

<div><p>More than 80 excited electronic states of the hydrohelium ion HeH⁺ of ^{1, 3}Σ⁺, ^{1, 3}Π, ^{1, 3}Δ, ^{1, 3}Φ and ^{1, 3}Γ symmetry have been calculated <i>ab initio</i> up to <i>n</i> = 6 for internuclear distances ranging from 0.5 to 100 bohr. The computations involve a configuration interaction (CI) treatment based on a home-made suite of programs that uses special basis sets designed for the representation of molecular Rydberg states. The results are compared with previous computations where these are available (up to <i>n</i> = 4), and it is found that except for the very lowest excited states, the present energies are consistently lower than those obtained previously, with an average lowering corresponding to several hundred cm⁻¹. It is shown that with the exception of its ground state, HeH⁺ is an effective one-electron system having an overall electronic structure similar to H⁺₂. The interaction of the excited electron with the He⁺ 1<i>s</i> core electron causes small singlet–triplet splittings to appear and ℓ-mixing interactions to occur, that are not present in H⁺₂.</p>