The interaction between first excited state hydrogen atoms and a different non-degenerate atom: an example of second-order resonance forces

The interaction of a first excited state hydrogen atom with a different non-degenerate atom is considered in the R ^-1 multiple approximation. The interaction energies for the Π states arising from this interaction have the form usually associated with second-order dispersion energies while those for the Σ states behave quite differently as a function of R. Because of a second-order ‘resonance within one molecule effect’ the second-order Σ-state energies do not possess single R ^-1 expansions that are formally valid for all values of R consistent with the multipole treatment. The expansions of these Σ-state energies, that are valid for ‘small’ R, contain terms varying as both even and odd powers of R ^-1; the lead R ^-7 ‘odd’ term competes strongly with the usual R ^-6 dispersion energy. The expansions that are valid for ‘large’ R contain terms varying as even powers of R ^-1 only and usually have little physical meaning. The He(¹S)-H(n=2) interaction is considered as a specific example of these results and the interaction energies for this system are evaluated through O(R ^-8) by using one-centre pseudostate methods. The general significance of the results is also discussed briefly.