Electron-Capture Processes of 10-40-MeV Oxygen Ions in Helium, Nitrogen, and Argon

Experimental data on electron-capture cross sections are analyzed in order to obtain the cross sections for removal of $n$ given atomic electrons, where $n=1, 2, or 3$. Probabilities of capture averaged over certain regions of impact parameter are obtained from ratios of these cross sections. Maxima are found in the capture probabilities for +8 oxygen ions in helium, and for +8 and +7 ions in nitrogen and argon. No maxima are found for ions of lower charge. The large cross sections at high velocities in nitrogen and argon are attributed to capture from the $K$ shell of nitrogen and the $L$ shell of argon, which contain electrons with momenta great enough to satisfy conservation requirements. The larger cross sections for +8 ions and the maxima in capture probability are attributed to exoergic capture from atomic energy levels into the deeply bound $K$ shell of the oxygen +8 ion. A criterion based on an atomic-orbital theory is given for the velocity at which maxima due to exoergic capture should occur, and this is in rough agreement with the observed velocities. Electron-capture cross sections from atoms with tightly bound shells can generally be expected to be large for heavy ions even at velocities greater than ${10}^9$ cm/sec.