State-selective electron-capture cross section measurements for low-energy collisions of He-like ions on H-2

We measured state-selective absolute electron capture cross sections for collisions of He-like C4+, N5+ and O6+ ions with molecular hydrogen by means of photon emission spectroscopy at impact energies ranging from 4 keV amu(-1) down to energies as low as 5 eV amu(-1). In this low-energy region the total cross sections are found to be weakly dependent on the projectile energy, whereas the state-selective ones change drastically with varying impact energy. Up to now no experiments were available to benchmark theoretical predictions. In our method we combine for the first time decelerated projectile beams with the possibility of measuring state-selective electron capture.

Compared with existing measurements for state-selective cross sections our deduced cross sections are in perfect agreement in the overlapping energy region (E much greater than 100 eV amu(-1)). Good agreement is also found on comparing our results with existing total cross sections. In comparison with theoretical calculations some remarkable and unexpectedly large discrepancies of orders of magnitude are found towards lower impact energies.