Single and double ionization of helium by intermediate-to-high-velocity He+ projectiles.

Abstract

Ionization of helium target atoms by ${\mathrm{He}}^{+}$ projectiles at energies of 0.125 to 3.0 MeV/u (v=2.2--11 a.u.) has been investigated. Ratios of double-to-single ionization are determined for projectiles undergoing no charge change, electron capture, or electron loss, respectively, and cross sections for target single and double ionization are determined for projectiles undergoing no charge change or electron capture. Total cross sections for single-electron capture and loss by the incident ${\mathrm{He}}^{+}$ projectiles are also determined. The present measurements extend to higher velocities than earlier results obtained by other investigators. As found by other investigators, the double-to-single-ionization ratios associated with projectiles undergoing electron capture or loss are significantly larger than those associated with no projectile charge change. The effective charges for target single and double ionization by ${\mathrm{He}}^{+}$ projectiles are determined by normalizing to single- and double-ionization cross sections, respectively, for ${\mathrm{H}}^{+}$ projectiles. For direct target ionization, i.e., ionization associated with no projectile charge change, the double-to-single-ionization ratios for ${\mathrm{He}}^{+}$ and ${\mathrm{He}}^{2+}$ projectiles are nearly equal at equal velocities. This implies that the cross sections for single and double ionization by ${\mathrm{He}}^{+}$ are altered by the same factor when compared to fully stripped ${\mathrm{He}}^{2+}$, a result which is consistent with the fact that the effective charges for single and double ionization are about the same.