Excitation and charge transfer to 2s and 2p states in 1--20-keV H+-H collisions

Electron excitation and transfer to 2s and 2p states in H+-H 1s collisions is studied in the energy range from 1 to 20 keV with the use of a modified two-center atomic-expansion method. It is shown that the inclusion of united-atom orbitals, in addition to the atomic orbitals of the separated atoms, in a two-center expansion allows for the extension of this method to the low-collision-energy regime. The results of our calculations at low energies from 1 to 5 keV agree with experiments and with the recent multistate molecular-orbital calculations by Kimura and Thorson but differ from the molecular-orbital calculations by Crothers and Hughes. At higher energies our results agree qualitatively with experiments and confirm the dip in 2s and 2p excitation cross sections at Eapprox.11 keV. The impact-parameter dependence of excitation and capture probabilities is also examined to illustrate the evolution of the excitation mechanism from the rotational coupling at lower collision energies to the direct excitation process at higher energies