Stopping of swift antiprotons by hydrogen atoms and the Barkas correction

We report calculations of the stopping cross sections of hydrogen atoms for protons and antiprotons at low to intermediate energies and take the difference explicitly to determine the Barkas correction for this system. The calculational method used is the electron-nuclear dynamics formalism which involves the coupled direct dynamics of all nuclei and electrons and thus includes all terms in the Born expansion. The formalism is a nonperturbational, ab initio approach to solve the time-dependent Schrödinger equation, applicable to all the projectile energies under consideration. This is in contrast to the use of different velocity-dependent models for different energy ranges used in other approaches. We find that at high projectile energies, target excitation and ionization are responsible for the projectile energy loss. However, at low projectile energies, the repulsion of the negatively charged projectile and the target electronic structure and its coupling to the target nuclei produce a billiard ball effect which combined with the large ionization and excitation induced by the antiproton is responsible for the large nuclear stopping power, contrary to near-adiabatic dynamics predicted by other models.