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Two-electron capture with emission of one photon in fast collisions between a highly charged ion and a light atom

We consider the electron capture process in fast non-relativistic ion–atom collisions in which the transfer of two electrons from the atom to the ion is accompanied by emission of one photon with the mean energy ω(2)k about two times larger than that ω(1)k characteristic for the radiative capture of one electron. Such a photon can appear both due to the uncorrelated capture process, in which two electrons are transferred to the ion independently via the non-radiative and radiative capture channels, and due to the correlated two-electron capture, where the electron–electron interaction plays the crucial role. The uncorrelated capture produces a photon spectrum which has a maximum at ω(1)k and gives the main contribution to the two-electron capture. The correlated capture mechanism leads to very small capture cross sections but produces a photon spectrum having a maximum at ∼ω(2)k which in principle enables separation of this process in an experiment.