Autoionization decay from doubly excited states of ${\mathrm{Ne}}^{6+}[1s^{2}3lnl$ ($n=3,4,5$)] (symmetric configurations) as well as the Coster-Kronig transition from doubly excited states of ${\mathrm{Ne}}^{6+}\left[1s^{2}2pnl(n⩾7)\right]$ (asymmetric configurations) are observed in the transfer ionization reaction channel of 80 keV/u ${\mathrm{Ne}}^{8+}$-He collisions. It has been predicted that the formation of symmetric configurations results from uncorrelated double-electron capture processes [Z. Chen and C. D. Lin, Phys. Rev. A 48, 1298 (1993)], and the formation of asymmetric configurations probably results from correlated double-electron capture caused by the dynamical electron-correlation effects which are attributed to small internuclear distances. However, previous experimental measurements were not able to obtain information about the dependence on internuclear distance which can be reflected in the transversal recoil-ion momentum. In this work, we measure the recoil-ion momentum in coincidence with the ejected electron velocity and find that the observed transverse recoil-ion momentum is smaller for the formation of symmetric ${\mathrm{Ne}}^{6+}\left[1s^{2}3lnl(n=3,4,5)\right]$ states than the formation of asymmetric ${\mathrm{Ne}}^{6+}\left[1s^{2}2pnl(n⩾7)\right]$ states. Since large momentum transfer occurs for small internuclear distances (strong electron-electron interactions) and small momentum transfer occurs for large internuclear distances (weak electron-electron interactions), the results indicate that dynamical electron correlation is important for the formation of the asymmetric states.

• Received 10 July 2015

DOI:https://doi.org/10.1103/PhysRevA.93.032709