Ionization and Electron Transfer in Collisions of Two H Atoms: 1.25-117 keV

Cross sections for electron loss and electron capture by H atoms in encounters with H target atoms are measured in the energy range 1.25 to 117 keV. The electron loss process is measured for H on ${\mathrm{H}}_2$ as a check on the method. Three methods of controlling the excited-state distribution of the H atom beam are investigated in the energy range 25 to 100 keV. One method achieves a nearly pure $1s$-state beam. Results are compared with theoretical Born-approximation calculations by Bates and Griffing; the theoretical cross section for the ionization process is about 0.5 times the experimental value at 20 keV and equal to the experimental value at ∼80 to 100 keV. The experimental values of the electron-capture cross section are a factor of 2.5 to 3 below Mapleton's Born calculations. The quantum-number dependence of the ionization cross section is deduced from a comparison of cross-section measurements obtained with the different beam-preparation methods. If the ionization cross section is assumed to vary as $n^{\alpha }$, where $n$ is the quantum number of the excited projectile atom and $\alpha$ is a constant, it is found that $\alpha$ lies between 0 and 1, with most probable values ${\mathrm{H}}_2$ and