Double electron capture in fast ion-atom collisions

A four-body formalism of the modified target continuum distorted wave (MTCDW-4B) with incorrect boundary conditions and boundary corrected continuum intermediate state (BCCIS-4B) approximation have been employed to calculate the differential cross sections (DCS) and total cross sections (TCS) for double-electron capture (DC) in collision of fast bare ions with helium atoms in their ground states. In both these formalisms, the intermediate continuum state of each of the active electrons with the target has been taken into account. The influence of the static-electron-correlations on cross sections has also been taken into account by choosing the proper wave functions of the initial states of the bound electrons. Moreover, we have computed the cross sections using the asymptotic Coulomb logarithmic phase for the relative motion of two colliding nuclei in BCCIS-4B theory. The present computed results are compared with the available experimental and other existing theoretical results. TCS are found to be in good agreement with the measurements. In addition, we have also analyzed DCS for DC in the collision of alpha-particles with helium atoms at intermediate and high projectile energies. We have investigated the significance of the contributions to TCS and DCS from excited states (both single and double) of He and Li+ especially in comparison between theories and measurements. For symmetric collision, the ground state capture dominates over the excited states whereas for asymmetric collision, the excited states including singly and doubly excited states dominate over the ground state capture. It is also clear that the ground state contribution dominates at very high impact energies for asymmetric collision. The obtained results for DCS into the ground state and excited states are compared with the experimental data and overall satisfactory agreements have been found at different impact energies.