Radiative double-electron capture (RDEC) involves the transfer of two electrons with the simultaneous emission of a single photon. This process, which can be viewed as the inverse of double photoionzation, has been studied for $2.11\mathrm{MeV}/\mathrm{u}$ ${\mathrm{F}}^{9+}$ and ${\mathrm{F}}^{8+}$ ions striking gas targets of ${\mathrm{N}}_2$ and Ne. The existence of RDEC is conclusively shown for both targets and the results are compared with earlier ${\mathrm{O}}^{8+}$ and ${\mathrm{F}}^{9+}$ findings for thin-foil carbon and with theory. The data for the carbon target showed some evidence for the existence of RDEC, but the interpretation was clouded by high-probability, unavoidable multiple collisions causing the exiting charge state to be increased.

• Received 29 March 2019
• Accepted 10 February 2020

DOI:https://doi.org/10.1103/PhysRevLett.124.133401