Electron capture from lithium and its ions by high-energy protons

Abstract

The continuum-distorted wave (CDW) method is used to calculate cross sections for electron capture by fast protons from the lithium atom and its related positive ions. With the exception of Li2'(12S), each target electronic state is described by a Hartree-Fock wavefunction. The capture states considered are (nl) = Is, 2s and 2p and the proton energy range is 200 keV E < 10 MeV, the lower limit being chosen to be roughly appropriate for a CDW description of capture from the lithium ions. For the lithium atom, capture can occur from either the K or L shell and the resulting ion can exist either in its ground state 1's or in one of the excited states 2% or 23S; cross sections are tabulated for each of these three states for capture into the above hydrogen levels (nl). Results for the individual capture states (nl) are also reported for the Li' target. Total capture cross sections are calculated for each target by means of the Oppenheimer n-3 rule. For Li and Li', total cross sections are also evaluated at E < 200 keV in order that graphical comparisons can be made with the limited experimental data. A procedure for determining the effective screening of the target nucleus by the uncaptured or 'passive' electrons is discussed and the sensitivity of the cross sections with respect to such screening is examined.