Electron capture and excitation (de-excitation) processes in slow collisions
of protons with excited helium, H+ + He*(1s, NL) → H*(nl) + He+(1s) or
→ H+ + He*(1s, N'L'),
are studied by using the close coupling of the states {N, n} = 2, 3, 4
within the approach described in a previous paper. At small collision
velocities, 106 cm s−1 < v < 5 × 107 cm s−1,
the cross sections for excitation (de-excitation) are very large, due to the
importance of two-step transitions. The coupling of the states with different
principal quantum numbers significantly increases the cross sections for electron
capture at adiabatically small collision velocities.
It is shown that two parallel ion beams react at a rate that is independent of their density profiles when made to oscillate against each other in a two-dimensional scanning motion. An experimental set-up that makes use of this principle is described. Absolute cross sections obtained in this way are in good agreement with those obtained with the beams merging in the usual (static) mode. Cross sections for single-charge transfer between and in the energy range 5 - 4000 eV are presented and compared to other existing data.
The electron-capture and excitation processes in slow collisions of protons with He * (1s3l) are studied using the close-coupling method within the semiclassical approximation. The Stark splitting of electron-capture states on H is explicitly taken into account and the coupling-matrix elements between these states and the initial angular-momentum states on He are calculated analytically. The cross sections for excitation (de-excitation) and single-electron capture to specific spherical hydrogen states have been calculated in the relative velocity range 2 × 10 6 -1.3 × 10 8 cm s −1 . The cross section values for both types of processes in the considered velocity range are found to be large (10 −14 -10 −13 cm 2 ) due to the large values of electron-exchange couplings at large internuclear distances. The excitation (de-excitation) processes are controlled by two-step exchange (capture and re-capture) transitions rather than by direct coupling among the states centred on He.
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