  The stopping power of protons and deuterons in low energy collisions with
helium gas targets is investigated with the numerical solution of the
time-dependent Schr\"odinger coupled-channels equations using molecular orbital
wavefunctions. It is shown that at low projectile energies the energy loss is
mainly due to nuclear stopping, charge exchange, and the excitation of low
energy levels in the target. The second and third mechanisms, called electronic
stopping, dominate for 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.
