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.