Triple electron capture in fast 0.5–1.1 MeV/u ${\mathrm{C}}^{6+}$ on Ar collisions

Doubly excited KLL states were populated by triple electron capture in collisions of fast $(v=4.5\mathrm{}–6.6\mathrm{a}.\mathrm{u}.\mathrm{})$ ${\mathrm{C}}^{6+}$ ions with Ar atoms. Measurements of the Auger-electron emission in the direction of the ion beam were used to determine the absolute single differential cross sections for the triple electron capture to all autoionizing KLL states. The results were compared with cross sections calculated within the independent-particle model, in which the simultaneous capture of all three target electrons was assumed. Single electron capture probabilities, employed by the model, were calculated using the two-center semiclassical close-coupling method, based on an atomic orbital expansion. In order to allow comparison of the measured zero-degree differential cross sections with calculated total cross sections, the Auger-electron emission from the doubly excited KLL states was assumed isotropic. Model calculations were found to be in a good agreement with the experimental data. An adequate description of the triple electron capture by the model implies that the projectile screening and electron-electron correlation effects in multiple electron capture are significantly reduced in fast, highly charged ion-atom collisions.