  he velocity dependence of the stopping power of swift protons and deuterons
in low energy collisions with hydrogen and helium gas targets is investigated
with the numerical solution of the time-dependent Schroedinger coupled-channels
equations using molecular orbital wavefunctions. At low projectile energies the
stopping is mainly due to nuclear stopping, charge exchange of the electron,
and excitation of the lowest levels in the target. The second and third
mechanisms dominate at E < 200 eV. At lower energies it is also shown that a
threshold effect is responsible for a quick drop of the energy loss. This
investigation sheds more light on the long standing electron screening problem
in fusion reactions of astrophysical interest.
