Molecular treatment of electron capture by protons from the ground and excited states of alkali-metal atoms

Electron-capture cross sections for H⁺ plus alkali-metal atom (Na, K, Rb, and Cs) systems have been computed for projectile energies from 10 eV to 10 keV. An impactparameter perturbed-stationary-state theory using molecular states that incorporate electron translation factors was used to calculate the cross sections. The wave functions were generated by employing the pseudopotential method. These yield equilibrium parameters R_e and D_e for the A²Σ⁺ molecular state that are in good agreement with ab initio results. Interaction energies are also presented for the LiH⁺ system. Basis sets of up to eight molecular states were used to calculate the electron-capture cross sections from ground (ns) as well as from the first excited (np) states of the alkali-metal atoms. Results for electron capture from the ground-state alkali-metal atom are in good agreement with the recent experiments of Nagata. Electron capture from excited alkali-metal (np) atoms does not yield enhanced cross sections relative to capture from the ground state and, in fact, shows decreased cross sections for the heavy alkali-metal atoms. Such behavior is contrary to predictions made using arguments based on the magnitude of the energy gap ΔE to the electron-capture product states.