Identification of distinct two-electron transfer processes in O6++Na collisions

Two-electron capture in 7.5 keV amu(-1) O6+ + Na collisions is investigated by measuring the momenta of the Na2+ recoil ions. The Q-value spectrum shows two groups of two-electron capture channels, namely 3l3l' and 3lnl' ( n >= 6). The manifolds in between, 3l4l' and 3l5l', are barely populated. The population of the 3lnl' ( n >= 6) states occurs via two consecutive one-electron transitions, while transfer to the 3l3l' channels occurs at a direct crossing with the entrance channel. These findings show that for capturing two non-equivalent electrons the population of symmetric and asymmetric nln'l' projectile states can also be described by dielectronic and monoelectronic transitions, respectively, just as for the capture of two equivalent electrons from noble gasses. However, the population mechanisms are the other way round.