ELECTRON CAPTURE OF He$^{2+}$ IONS FROM ATOMIC AND MOLECULAR HYDROGEN H.F. Busnengo$^\diamond$, S.E. Corchs$^\diamond$, R.D. Rivarola$^\diamond$, M.M. Sant'Anna$^*$, W.S. Melo$^*$, A.C.F. Santos$^*$, G.M. Sigaud$^*$, E.C. Montenegro$^*$, W.E. Meyerhof$^\dagger$, M.B. Shah$^+$ and L. Guly\'as$^\ddagger$ $^\diamond$ Instituto de F\'{\i}sica Rosario (CONICET-UNR), 2000 - Rosario, Argentina $^*$ Departamento de F\'{\i}sica, PUC, Rio de Janeiro 22452-970, Brazil $^\dagger$ Department of Physics, Stanford University, Stanford, CA 99305-4060 $^+$ Department of Pure and Applied Physics, The Queen's University, Belfast BT7 1NN, Northern Ireland $^\ddagger$ Institute of Nuclear Research, Hungarian Academy of Sciences, H-4001, Debrecen, Hungary Single electron capture by impact of alpha particles on H and H$_2$ targets is studied both theoretically and experimentally. The relative importance of {\it rearrangement} and {\it direct} processes on the cross section ratio is determined. A hump on the cross section ratio at intermediate collision energies may be attributed to two electron processes where the projectile impacts sequentially each one of the electrons of the target.