Charge-changing cross sections for 1--70-keV H+ and H0 in collisions with calcium and strontium metal vapors

Absolute electron-transfer cross sections have been measured for H+ and H0 in collisions with Ca and Sr metal vapor targets for energies between 1 and 70 keV. The measurements include cross sections for single- and double-electron capture by H+ and single-electron capture and loss by H0. The single-electron capture cross section for H+ incident has been calculated using the binary-encounter approximation, and agrees with experiment to within a factor of 2 for energies between 2 and 70 keV. The electron-detachment cross section for H- in collisions with Ca and Sr is determined from the present measured data and previously measured charge-state equilibrium fractions. The high- and low-energy structure in the cross sections for single- and double-electron capture by H+ are discussed in terms of the importance of inner-shell capture at high energies and a molecular picture at low energies, and the cross sections are compared to those for Mg and Ba targets published in a previous paper. The cross section for single-electron capture by H0 rises at low energies; that for electron detachment from H- decreases at low energies in agreement with theoretical predictions of Olson and Liu. This situation is discussed as the mechanism responsible for large negative-ion equilibrium yields at low energies for alkaline-earth targets