The electron transfer mechanism was investigated for the single-electron-capture process in 3-keV/u ${\mathrm{Ar}}^{8+}$-He collisions with cold target recoil ion momentum spectroscopy. The different state-selective electron capture processes were identified for one $1s$ electron of He capture into $4s$, $4p$, $4d\text{−}4f$, and $5s\text{−}5p$ states of ${\mathrm{Ar}}^{8+}$. We observed that $1s$ to $4s$ capture mainly happens at small transverse recoil ion momentum, while $1s$ to $4p$ capture mainly happens at large transverse recoil ion momentum. By comparing the measured projectile scattering angle distributions to the theoretical calculations, it is suggested that $1s$ to $4s$ and $1s$ to $4p$ state-selective capture processes are mainly caused by radial and rotational coupling effects, respectively, and the $4p_{\pm 1}$ quantum states are preferred.

• Received 11 November 2016

DOI:https://doi.org/10.1103/PhysRevA.95.042702