An (e,2e)–(e,3e) investigation of argon: competition between inner-shell single ionization and direct double ionization processes

We report new (e,2e) and (e,3e) experiments for ionization of argon under unexplored kinematics characterized by large energy transfer to the target and where the two 'atomic' electrons carry the same kinetic energy. This opens the possibility of competition and interference between direct 3p-outer shell double ionization (DI) and indirect DI via an Auger emission from the 2p shell. The (e,2e) results are found to be in reasonably good agreement with state-of-the-art theoretical models for 2p-single ionization. The importance of the other competing processes, namely direct 3p-DI and/or Auger relaxation, not included in the models is highlighted. The measured (e,3e) angular correlation pattern displays a rich structure, partly resulting from final state interactions where the outgoing electrons tend to minimize their Coulomb repulsion. The rapprochement with the (e,2e) results allows the assignment of some of the observed peaks to given ionization mechanisms, either the two-step 2 direct 3p-DI, or a 2p-single ionization followed by an Auger emission.