Using measured cross sections, empirical scaling laws are extracted for projectile stripping induced by collisions with a many-electron target. By scaling both the cross sections and the impact velocities, it is shown that a single universal curve can be used to fit data for single and multiple electron loss from negative ions, neutral particles, and singly or multiply charged positive ions. The scaling applies to projectiles ranging from hydrogen to uranium and collision energies ranging from below $\text{keV}∕\mathrm{u}$ to hundreds of $\text{MeV}∕\mathrm{u}$. At high energies, existing data are consistent with a $v^{-1}$ impact velocity dependence for scaled velocities less than 10. Limited data imply that above 10 the velocity dependence becomes $v^{-2}$. Using our universal curve, cross sections are predicted for electron loss from low-charge-state heavy ions at 20 and $100\text{MeV}∕\mathrm{u}$.

Experimental data are presented for differential electron emission from helium induced by isotachic ${\mathrm{H}}^{+}$ and ${\mathrm{He}}^{2+}$ impact. Electron emission at 15° was studied in order to investigate maxima in the ${\mathit{d}}^2$σ/${\mathit{dv}}_{\mathit{e}}$dΩ spectra. These maxima were previously observed to shift to lower electron velocities as the projectile charge state increased and the shifts were interpreted as evidence of an independent saddle-point-ionization mechanism. The present data for ${\mathrm{H}}^{+}$ and ${\mathrm{He}}^{2+}$ impact do not confirm any such shifts and indicate that previous ${\mathrm{He}}^{2+}$ measurements, where shifts were observed, are in error.

Doubly differential cross sections for single and multiple ionization of Ar have been measured for 500, 750, and 1000 eV electron impact. The cross sections were measured as a function of projectile energy loss and scattering angle. The energy loss range was 0–85 % of the initial projectile energy and scattering angles were between $\pm 22°.$ The data were put on an absolute scale by normalizing to total ionization cross sections available in the literature and found to be in good agreement with the absolute electron impact cross sections from DuBois and Rudd. For 750 eV impact, a comparison was made between the present electron impact data and positron impact data obtained using the same experimental conditions. The same energy dependence and yields for single ionization were found for both electron and positron impact. On the other hand, the double- and triple-ionization yields are smaller for positron impact as compared to electron impact. Comparisons with photoionization data showed that for outer shell ionization the fractions of double and triple ionization of argon by photon impact are in quite good agreement with the present electron impact data.

A scaling for the total ionization cross sections of atomic and molecular targets by neutral to highly charged projectiles with energies ranging from a few keV/u to many MeV/u is presented, with few free parameters, all velocity independent, and whose values do not vary appreciably for the projectiles and atomic and molecular targets considered here. Another important feature of the proposed scaling is that it has been built considering the characteristics of the collision systems, taking into account the various physical processes that lead to target ionization and the velocity regions where each of them dominates, and performing a smooth transition between them. This scaling describes quite well the existent experimental data covering a broad range of collision systems as diverse as H${}^{+}$ on H, Xe${}^{30+}$ on He, or O${}^{6+}$ on uracil. DOI: http://dx.doi.org/10.1103/PhysRevA.87.012706 ©2013 American Physical Society

Triply differential electron emission cross sections are measured for single ionization of argon by 500 eV positrons. Data are presented for coincidences between projectiles scattered into angles of 3° and electrons with emission energies less than 10 eV that are observed between 45 and 135° along the beam direction. For interpretation, these are compared to cosine squared representations of the binary and recoil lobes which are convoluted over experimental parameters. Singly differential electron emission data for double and triple ionization by positrons are also presented.