Cusp-electron production in collisions of open-shell He-like oxygen ions with atomic targets

We report on double differential cross sections of cusp electrons obtained in MeV/u collisions of open-shell He-like oxygen ions with helium. We use zero-degree electron spectroscopy and our double measurement technique, involving ${\mathrm{O}}^{6+}\left(1s^2\right)$ ground-state and ${\mathrm{O}}^{6+}(1s^2,1s2s)$ mixed-state beams, to extract the cusp contribution from only the excited ${\mathrm{O}}^{6+}\left(1s2s\right)$ beam. Theoretical calculations based on the continuum distorted wave eikonal initial state are in very good agreement with the measurements. The roles of the processes of electron capture to the continuum, electron loss to the continuum, and electron loss to the continuum with simultaneous target ionization, which contribute to the cusp peak, are discussed in detail.

Figure 1

KLL Auger spectra measured at zero degrees with respect to the projectile velocity for collisions of 24-MeV ${\mathrm{O}}^{6+}(1s^2,1s2s)$ with He gas targets. The red solid circles correspond to a mixed-state beam with a high-value fraction, $f_{1s2s}=(23\pm 4)\%$, while the black solid squares correspond to a very low-value fraction, $f_{1s2s}<2\%$, i.e., to a practically pure ground-state beam.

Figure 2

Zero-degree DDCS of cusp electrons measured in collisions of 24-MeV ${\mathrm{O}}^{6+}$ with He gas targets. Red solid circles: Mixed-state ${\mathrm{O}}^{6+}(1s^2,1s2s)$ beam with $f_{1s2s}=23\%$. Black solid squares: Ground-state ${\mathrm{O}}^{6+}\left(1s^2\right)$ beam.

Figure 3

CDW-EIS calculations for zero-degree DDCS of cusp electrons for collisions of (a) 24-MeV ${\mathrm{O}}^{6+}\left(1s^2\right)$ and (b) 24-MeV ${\mathrm{O}}^{6+}\left(1s2s\right)$ with He gas targets. Black solid lines: ECC contribution. Red short dashed line: ELC from the $1s$ electron. Blue dash-dotted line: ELC from the $2s$ electron. Green short-dotted line: Total ELC with simultaneous He target single ionization.

Figure 4

Zero-degree DDCS of cusp electrons for collisions of 24-MeV ${\mathrm{O}}^{6+}\left(1s2s\right)$ with He gas targets. Blue solid circles: Experimental data obtained from the double measurement cusp data shown in Fig. 2. Red line: Calculations based on the CDW-EIS theory.