Velocity dependence of electron-capture partial cross sections and alignment in low-energy collisions of ${\mathrm{Ne}}^{8+}$ and ${\mathrm{Ar}}^{8+}$ with atomic Na

We have measured the intensities and polarizations of fine-structure lines in the N=9-8 manifolds of Ne viii and Ar viii spectra produced in collisions of ${\mathrm{Ne}}^{8+}$ and ${\mathrm{Ar}}^{8+}$ (v=0.05–0.15 a.u.) with atomic sodium, and determined alignment and partial cross sections for electron capture as functions of projectile velocity. At low velocity the partial cross section for capture into the S levels is significantly higher than for capture into higher L levels, and at higher velocities the higher-L partial cross sections increase for Ar viii and dominate for Ne viii. The low-L-level cross sections are in fair agreement with multichannel-Landau-Zener calculations, while the high-L-level cross sections are in qualitative disagreement. For the neon projectiles, the polarization of the Bohr line is approximately 0.3 and independent of velocity; for argon projectiles, the polarization is approximately 0.15 and increases with increasing velocity. A qualitative theoretical interpretation is offered.