The Fermi model as a key for understanding excited state potential energy curves

The interaction of a Rydberg electron with a ground-state atom is modelled using the Fermi approximation to obtain long-range potentials for various excited diatomic molecules. Particular attention is paid to the cases of degenerate and near-degenerate atomic Rydberg levels. Extensive comparisons are performed with recent ab initio calculations for (1)Sigma(+) and (3)Sigma(+) states of NaH and He-2, for up to the ninth state of a specified symmetry in the former. Overall, good qualitative and in some cases, quantitative agreement is found. For isolated states, the undulations in the potentials clearly reflect the nodal pattern of the Rydberg state involved and the magnitude is fixed in the Fermi approach by the electron-ground-state atom scattering length. When several atomic states are close in energy, in the Fermi model because of the factorizable nature of the interaction, only a single non-zero potential emerges from the remaining flat manifold. Such behaviour explains, for example, the presence of high barriers at large distances seen in ab initio calculations for the (1,3)Sigma(+) 4f states of NaH. For NaH, the Fermi results are generally better for the weaker triplet interactions than the singlets. In case of near-degenerate levels, the weaker interactions are normally described better than the stronger interaction, where avoided crossings (not included in this simple model) may be important. The Fermi model is also shown to yield a physically sound diabatic approach for excited states of diatomic molecules, allowing for a deeper understanding of the unusual shapes of the adiabatic curves. Limitations of the Fermi model are also discussed. (C) 2002 Elsevier Science B.V. All rights reserved.