Observation of Radiative double-electron capture (RDEC) by F⁹⁺ions in collisions with single-layer graphene

Radiative double-electron capture (RDEC) occurs when the capture of two electrons by a fully stripped ion is accompanied by the simultaneous emission of a single photon. This process, fundamental in atomic collisions, is considered the inverse of double photoionization by a single photon. RDEC has been successfully studied with F⁹⁺ ions for a [LMDS(1] single-layer of graphene¹[LMDS(2] . A graphene target (∼0.35 nm thick) was mounted on a silicon nitride supporting grid (200 nm thick) consisting of ∼6400 holes of 2 µm diameter on a 200 µm thick hexagonal silicon substrate with a 0.5 x 0.5 mm aperture. For single-layer graphene, [LMDS(3] the foil thickness is close to that of a [LMDS(4] gas target and about a hundred times smaller than the thin-foil carbon. A Si(Li) x-ray detector placed at 90° to the beam[LMDS(5] detected the emitted x rays in coincidence with magnetically separated outgoing charged particles counted with silicon surface-barrier detectors. In the present work, the RDEC measurements have been repeated with single-layer graphene as well as a sample that had no graphene on it. X rays attributed to RDEC were seen for two separate graphene samples, while no RDEC x rays were observed for the sample without graphene, [LMDS(6] as expected. These measurements were just completed and analysis is underway, so the results will be reported later. Cross section values, which will be obtained from the data, will be compared with our previous gas target² and C-foil target³.