The experimental, unambiguous identification of underlying transition mechanisms in collision reactions often requires that measurements go beyond total cross sections to differential measurements. Low energy partial cross section measurements of electron capture for multicharged ions with atomic H are difficult and are not available at collision energies of a few eV/u. Using a simple multi-grid energy analyzer in a merged-beams apparatus, measurements of state-selective cross sections and angular distributions are now possible at eV/u energies. Measurements take advantage of the fact that the heat of reaction is amplified when kinematically transformed from the center-of-mass frame to the laboratory frame. This amplification results in a change in velocity of the collision products and increases with decreasing energy. The figure shown above left corresponds to the collision product D⁺ energy spectra for Si⁴⁺ + D at 50.6 and 98.6 eV/u. Fits to the spectra give the ratio of capture to the Si³⁺ 4s and 3d states and result in a discrepancy with coupled-channel molecular-orbital theory of nearly a factor of two. The figure above right corresponds to the angular distributions (calculated directly from the energy spectra) for collisions at 3.6 eV/u and 9.9 eV/u and shows that the angular scattering is increasing rapidly at these low energies. Such angular distribution measurements provide a new test of theory.

1. W. Wu and C. C. Havener, "Special Report", invited papers of XX ICPEAC, Vienna 1997.
2. W. Wu and C. C. Havener, J. Phys. B: At. Mol. Opt. Phys. 30 , L213 (1997).