Coupled-channels calculations of excitation and ionization in ion-atom collisions

Abstract

A numerical method has been used to compute excitation and ionization cross sections for ion-atom collisions. The projectile is treated classically and follows a straight line, constant velocity path (unless indicated otherwise). The wave function that describes the atom is expanded about the target in a truncated Hilbert space. The interaction between the projectile and the target atom is treated as a time dependent perturbation. A unitary time development operator, U, propagates the wave function from a time prior to the collision to a time after the collision in small time steps. Contrary to first-order theories, coupling between states is allowed. This method has been improved so that any number of partial waves can be included in the wave function expansion. This method has been applied to study negatively charged projectiles. Cross sections are obtained for collisions of antiprotons on atomic hydrogen (30 keV-372 keV) and compared with cross sections of protons on atomic hydrogen to explore the Z(,P) dependence. The antiproton-hydrogen results were converted into electron-hydrogen values with E(,e) = E(,P)(m(,e)/m(,P)) (15 eV-200 eV) and compared to experimental values. In addition, classical trajectories were also computed for electrons deflected from a straight line path by screened and unscreened interactions with the target nucleus. Polarization fractions were also computed. The method is then applied to study vacancy production from the L-shell. The partial wave convergence of the cross sections was carefully studied for s through g waves. The effect of using different potentials to describe the unperturbed target atom was also examined, particularly in regards to their effect on the binding energies. A modification of the Hartree-Fock potential is presented that leaves the wave functions and bound state orbital energies unaltered but lowers the continuum orbital energies and brings the binding energies (and ionization cross sections) closer to experimental values. With this potential collisions between protons (and alpha-particles) and argon are studied to explore the Z(,P) dependence of the cross sections. The cross section ratio (sigma)((alpha))/(4(sigma)(p)) is compared to experiment.