The experimentally observed enhancement of (1s2s 3s)ml4P to (1s2s3S)ml2P populations in collisions of He-like ions with H2 and He targets is discussed and explained by a state-selective cascade feeding mechanism. Continuum Distorted Wave (CDW) and Classical Trajectory Monte Carlo (CTMC) calculations show that large populations of higher lying (1s2s 3S)nℓ2'4L Rydberg states for ℓL0, n-1 and n 2 - 8 can be formed in 0.25-2 MeV/u collisions by direct capture to metastable F7+(1s2s3S) ions. These states subsequently decay through a cascade sequence of radiative or Auger transitions dictated by angular momentum and spin selection rules. All radiative (E1) and Auger transitions for n 5 were evaluated using the Cowan package for Hartree-Fock calculations. The (1s2s3S)ml2L doublets were found to Auger decay rapidly to the 1s2 level allowing for negligible cascade feeding of lower lying doublets including the 1s2s2p2P. Alternatively, Auger decay of the (1s2s3S)ml4L quartets is blocked by spin selection rules. Decay can proceed, however, through dipole transitions, but only to other quartets, including the lowest lying 1s2s2p 4P state, which in this way acts as an "excited ground state" eventually collecting most cascade flux. This selective cascade feeding mechanism thus leads to the non-statistical enhancement of the 1s2s2p4P quartet population over those of the 1s2s2p2P doublet populations providing an alternative explanation to the recently suggested novel mechanism of dynamical Pauli exchange.
ASJC Scopus subject areas
- Physics and Astronomy(all)