Z Dependence in Electron Capture by Protons.

Angular differential cross sections for H(2p) formation by 50 keV protons colliding with argon and neon have been measured. The measurements were made by detecting the neutral hydrogen atom after the collision in coincidence with the Lyman-alpha photon emitted when the H(2p) decays to the ground state. The measurements were normalized to known total cross sections for charge capture to the 2p state of hydrogen. Center-of-mass scattering angles range from 0 to 3.5 milliradians. The differential cross sections for H(2p) charge capture in proton collisions with argon, neon, and helium are compared. Although the total cross sections for capture to the 2p state vary widely, the angular distribution of the three differential cross sections are very similar. Only very near zero scattering angle is there much difference of the differential cross sections although the difference observed may not be statistically significant. The differential cross sections were transformed to probabilities as a function of impact parameter through a model potential which used a sum of screened Coulomb potentials to fit to the Hartree-Fock potential. Correlating the probability versus impact parameter with the radial probability density calculated from Hartree-Fock wave functions indicates the capture process occurs primarily from the outermost electron. Examination of the overlap in momentum space of the projectile and the target electrons gives further credence to the same conclusion.