Double- and single-electron capture and loss in collisions of 1<i>–</i>2-MeV/u boron, oxygen, and silicon projectiles with helium atoms

Single- and double-electron-capture cross sections were measured for 1--2-MeV/u bare ${\mathrm{B}}^{5+}$, ${\mathrm{O}}^{8+}$, and ${\mathrm{Si}}^{14+}$ projectiles in collisions with helium atoms. The measured double-electron-capture cross sections are 2--3 orders of magnitude smaller than corresponding single-capture cross sections. The single-capture cross sections are well described by continuum-distorted-wave calculations. For ${\mathrm{B}}^{5+}$, the double-capture cross sections at high velocities are in reasonable agreement with scaled Oppenheimer-Brinkman-Kramers calculations, assuming capture of independent electrons. Cross sections for electron capture and loss are also reported for nonbare ions (${\mathrm{O}}^{6+}$, ${\mathrm{O}}^{7+}$, ${\mathrm{Si}}^{8+}$, and ${\mathrm{Si}}^{13+}$). Single-capture cross sections for nonbare ions are about 30% smaller than corresponding cross sections for bare ions with the same ionic charge q. The reported electron-loss cross sections are well described as ionization of an ion by an incident helium nucleus and two free electrons, using theoretical ionization cross sections in the Born approximation.