Evidence for radiative double-electron capture (RDEC) by F⁹⁺ions in collisions with single-layer graphene

Evidence for radiative double-electron capture (RDEC) has been observed for F⁹⁺ ions in collisions with single-layer graphene. To our knowledge this the first such experiment for RDEC with this novel target. RDEC occurs when a highly-charged ion captures two electrons from a target while simultaneously emitting a photon and can be considered the time inverse of double photoionization. The experiment was done at Western Michigan University using a 40 MeV F⁹⁺ beam to interact with the graphene. X rays were observed at 90^o to the incident beam in coincidence with the outgoing ions after separation with a dipole magnet. Selecting the graphene target was a challenge, resulting in using a ~0.35 nm thick graphene sample mounted on a 200 nm thick silicon nitride supporting grid with ~6400 holes of 2 mm diameter. This entire assembly (purchased commercially) was mounted on 200 mm thick 3.0 mm hexagonal silicon substrate with a 0.5 x 0.5 mm aperture. X rays attributed to RDEC were seen for three separate graphene samples and the results are compared with our previous gas target¹ and C-foil target² results. For single-layer graphene, the foil thickness is close to that of the gas target and about a hundred times smaller than the thin-foil carbon used. Despite this, surprisingly, the graphene produced results appearing similar to those of the foil target. *Supported in part by NSF Grant No. 1707467.  ¹D. S. La Mantia et al., Phys. Rev. Lett. 124, 133401 (2020).  ²D. S. La Mantia et al., Phys. Rev. A 102, 060801(R) (2020).