Single- and double-electron capture by 1-100-keV protons in collisions with magnesium and barium atoms

Absolute total cross sections for single- and double-electron capture by ${\mathrm{H}}^{+}$ in collisions with Mg and Ba have been measured for energies between 1 and 100 keV. The single-electron-capture cross section has also been calculated with the Gryzinski formalism of the classical binary-encounter approximation. The measured cross sections exhibit both high- and low-energy structure. The high-energy behavior is in accord with the Brinkman-Kramers results and classical predictions for the energy at which inner-shell capture becomes important. The low-energy behavior is discussed in terms of a molecular picture of the collision. The cross sections are shown to be critically dependent on curve-crossing effects which occur at avoided crossings between potential curves. At low energies the double-electron-capture cross section in Mg shows pronounced structure consistent with a description of the collision in terms of transitions via the intermediate H ($1s$) state. It is shown that this mechanism is not important in Ba, since curve crossings are expected to take place at internuclear separations too large for transitions to occur. A possible explanation for the decrease in single-electron-capture cross section in Ba at low energies is suggested in terms of competition from direct target-excitation collisions. For single-electron capture in Mg, recent low-energy perturbed-stationary-state calculations and high-energy classical-trajectory Monte Carlo calculations by Olson and Liu are in good agreement with experiment. The present classical binary-encounter approximation results for single-electron capture in both Mg and Ba agree with experiment to within a factor of 2 over the entire energy range. Measurements of single- and double-electron capture are also reported for Ar targets. A pronounced second maximum in the double-electron-capture cross section is found.