Radiative double-electron capture for oxygen and fluorine ions colliding with thin-foil C: Effects of multiple collisions

Radiative double-electron capture (RDEC), a process considered the inverse of double photoionization of ions, has been investigated for similar to 2 MeV/u fully stripped and one-electron oxygen and fluorine ions colliding with thin-foil C targets. These measurements are a follow-up to the first evidence for RDEC [A. Simon, A. Warczak, T. Elkafrawy, and J. A. Tanis, Phys. Rev. Lett. 104, 123001 (2010)] in ion collisions with carbon, and to our recent observation of the process for 2.11 MeV/u F-9,F-8+ ions [D. S. La Mantia, P. N. S. Kumara, S. L. Buglione, C. P. McCoy, C. J. Taylor, J. S. White, A. Kayani, and J. A. Tanis, Phys. Rev. Lett. 124, 133401 (2020)] in collisions with gas targets of N-2 and Ne. Coincidences between emitted photons and outgoing ions in charge states q-2 (the expected RDEC charge state), q-1, and q were recorded. Differences in coincidences with all three charge states are appreciable and are attributed to unavoidable multiple charge-changing collisions of the ions as they transverse the thin-foil target. Also, significant differences between the spectra for oxygen and fluorine are seen, despite these ions being just one atomic number apart. Cross sections for RDEC were determined and compared with previous data for thin-foil solid and gas targets as well as with theoretical calculations.