Ion-Rydberg atom collision cross sections

Classical-trajectory Monte Carlo calculations have been performed for collisions of ions in charge states q=+1, +2, +5 and +10 with hydrogenic atoms in principal quantum levels n=1, 2, 5, 10 and 20. The collision velocity range investigated was 1<or=v/ve<or=10 where ve is the orbital velocity of the Rydberg electron (1/n in atomic units). Both charge-exchange and impact ionisation cross sections were calculated with impact ionisation found to be the dominant channel for v/ve>or=2. For v/ve>or approximately=5, the sum of the charge-exchange (CEX) and impact ionisation (ION) cross sections may be represented by sigma CEX+ION(a02)=6 pi n2q2/v2, where v is in atomic units. Analysis of the electronic levels produced after charge exchange by the ion indicates the capture proceeds into excited levels which tend to preserve the energy and orbital size of the initial Rydberg atom.