Postcollision effects in target ionization by ion impact at large momentum transfer

We have measured and calculated fully differential cross sections for target ionization in 16-MeV O${}^{7+}$+He and 24-MeV O${}^{8+}$+Li collisions. As in previous studies, in the case of the He target we observe a pronounced forward shift in the angular distribution of the electrons relative to the direction of the momentum transfer q at small $q$ ($q$ < 1 a.u.). An unexpected result is that we also find a strong forward shift at large $q$ ($q$ > 2 a.u.), while at intermediate $q$ this shift becomes very weak or even turns into a backward shift. For the Li target, in contrast, the forward shift monotonically increases with increasing $q$. These observations are qualitatively reproduced by our calculations. The comparison to theory suggests that at large $q$ the forward shift is due to the postcollision interaction between the outgoing projectile and the ejected electron, but at small $q$ it is mostly due to an interplay between the projectile-target core interaction and the electron-target core interaction.

Figure 1

Fully differential cross sections for target ionization in 16-MeV O${}^{7+}$+He collisions for electrons with an energy of 8±2 eV ejected into the scattering plane. The momentum transfer is fixed at 0.5±0.1 a.u. (left panel), 1.5±0.2 a.u. (middle panel), and 4±0.5 a.u. (right panel). The dashed curve shows the CDW-EIS calculation and the solid curve the CDW-EIS-PT calculation.

Figure 2

Forward shift of the binary peak relative to the direction of q as a function of $q$ for 16-MeV O${}^{7+}$+He collisions. The dashed curve shows the CDW-EIS calculation, the solid curve the CDW-EIS-PT calculation, the dotted curve the FBA-PT calculation using a Coulomb wave for the ejected electron, and the dash-dotted curve the FBA-PT calculation using a plane wave for the ejected electron.

Figure 3

Fully differential cross sections for target ionization from the 2$s$ state in 24-MeV O${}^{8+}$+Li collisions for electrons with an energy of 8±2 eV ejected into the scattering plane. The momentum transfer is fixed at 0.5±0.1 a.u. (left panel), 1.5±0.2 a.u. (middle panel), and 2.5±0.5 a.u. (right panel). The dashed curve shows the CDW-EIS calculation and the solid curve the CDW-EIS-NN calculation.

Figure 4

Same as Fig. 2 but for 24-MeV O${}^{8+}$+Li collisions.