Electron emission in ionization of He and Ne by fast dressed oxygen ions and projectile-charge-state dependence

The double-differential cross sections (DDCS) of low-energy electrons emitted at forward, backward, and perpendicular directions are reported for collisions of 3.75 MeV/u ${\mathrm{O}}^{q+}$ ($q=5$, 6, 7, 8) projectiles with He and Ne targets. The measured DDCS are found to be deviating from the $q^2$ dependence throughout the entire energy region. The effect of projectile electrons, for the dressed ions, as a function of the impact parameter is clearly noticeable for large as well as low-impact parameter collisions. We also present a theoretical calculation based on the prior form of the continuum distorted wave–eikonal initial state approximation, in which the projectile-active electron interaction is modeled with the Green-Sellin-Zachor potential. This particular representation of the potential has been proven to give good qualitative results for projectiles with residual electrons. In addition to the total DDCS, the individual contributions from target ionization, projectile electron loss, and simultaneous ionization processes are also calculated. The total DDCS obtained from these calculations are shown to be in excellent agreement with the experimental observations.

Figure 1

DDCS energy distributions for He target at ${30}^{\circ },{90}^{\circ }$, and ${150}^{\circ }$. DDCS corresponding to ${\mathrm{O}}^{8+} \left({\mathrm{O}}^{5+}\right)$ is symbolized by a black triangle (red circle). CDW-EIS calculations are denoted as follows: (a) Black dashed line: DDCS corresponding to ${\mathrm{O}}^{8+}$; (b) red solid line: DDCS corresponding to ${\mathrm{O}}^{5+}$; (c) blue dotted line: target ionization; (d) brown dash-dotted line: target-projectile simultaneous ionization; (e) green dash-double-dotted line: projectile ionization. Inset: Same CDW-EIS plots extended up to higher electron energy.

Figure 2

Same as Fig. 1 for Ne target.

Figure 3

Energy distribution of DDCS ratio $\left({\mathrm{O}}^{5+}/{\mathrm{O}}^{8+}\right)$ at different angles for the He target. The CDW-EIS calculation is denoted by a red solid line (m), which is the ratio of total DDCS corresponding to ${\mathrm{O}}^{5+}$ and ${\mathrm{O}}^{8+}$. The blue dashed line (n) denotes the Born $\left(q^2\right)$ prediction. The green dotted line (o) (in ${150}^{\circ }$ plot) indicates the crossover point, i.e., ratio $=1.0$. Inset: Contributions from different ionization processes to the total DDCS ratio.

Figure 4

Same as Fig. 3 for Ne target for different $q$ values.