Cusp-electron production in collisions of mixed-state Be-like oxygen beams with helium targets

We report on double differential cross section (DDCS) measurements of cusp electrons emitted at zero degrees in swift collisions of mixed-state O ⁴⁺(1s²2s²\;¹\!S, 1s²2s2p\;³\!P) 4 + ( 1 s 2 2 s 2 1 S , 1 s 2 2 s 2 p 3 P ) ions with He gas targets. The content of the mixed-state beam is varied depending on the upcharging method, i.e. gas or foil stripping, and thus the DDCS spectra bear the signature of the contribution of both components of the mixed-state beam. Additionally, high resolution Auger spectra corresponding to different beam content were recorded allowing thus for the determination of the fraction of the 1s²2s2p\;³\!P 1 s 2 2 s 2 p 3 P part of the beam. Our DDCS measurements, involving four projectile electrons in open-shell configuration, provide stringent tests for current advanced collision theories such as the continuum distorted wave theories.