Direct reactions in relativistic atomic collisions and the influence of Coulomb boundary conditions

It is shown that in a Born treatment of excitation, ionization, and pair production in relativistic atomic collisions, cross-section calculations are not affected by the presence or absence of explicitly imposed Coulomb boundary conditions. On the other hand, in relativistic single- or two-center coupled-channel calculations, the results are strongly affected. For ${\mathrm{U}}^{92+}$+${\mathrm{U}}^{91+}$ collisions at 1 GeV/u, it is illustrated that in all calculations unperturbed atomic basis sets lead to long-range couplings along the trajectory while for basis states satisfying boundary conditions the couplings among states are localized around the distance of closest approach. The necessity of imposing Coulomb boundary conditions for relativistic direct reactions is emphasized.