Low-Energy State-Selective Charge Transfer by Multiply Charged Ions

We present a combined rf-guided ion beam and photon emission spectroscopy method, which facilitates state-selective charge-transfer measurements at energies of direct relevance for astrophysics and fusion-plasma diagnostics and modeling. Ion energies have been varied from 1000 eV/amu down to energies as low as 5 eV/amu. Absolute state-selective cross sections have been obtained for He ²⁺ and N ⁵⁺ ions colliding on molecular hydrogen. Orders of magnitude differences are found between theory and the present results. This indicates clearly that such data are valuable as benchmarks for the necessary advancement of theoretical descriptions.