Production of autoionizing states by double-electron capture in intermediate-energy C4+ + He collisions

We have performed a kinematically complete experiment for state-selective double-electron capture occurring in 15-keV/u ${\mathrm{C}}^{4+}$ $+$ He collisions by means of cold-target recoil-ion momentum spectroscopy. It was shown that capture into ground and low excited states of the ${\mathrm{C}}^{2+}$ ions is overwhelmingly dominant. Besides, a small fraction of autoionization decays from doubly excited states of symmetric as well as asymmetric electron configurations following endothermic double-electron capture were clearly observed. Emphasis was given to the population mechanisms of the different electron configurations. Our analysis indicates that electron-electron correlation effects may play a major role in the double-electron capture process. In addition, the large transverse recoil-ion momentum of symmetric configurations as compared to asymmetric configurations for the doubly excited autoionizing states may be attributed to the different number of steps involved in the primary capture process, with the former likely produced by a two-step mechanism and the latter by a one-step mechanism.