Electron Exchange in H-H Collisions: Heitler-London Curves as a Function of Internuclear Distance and Velocity

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Abstract
The Heitler-London curves for the interaction of two hydrogen atoms are calculated as a function of the internuclear distance and velocity. Electron-exchange terms are found to dominate the interaction energy at laboratory energy 1 keV, to be quite significant at 25 keV, and to be negligible at 400 keV. The exchange energy is attractive at 25 keV and below, but at 100 keV it is repulsive. Hence, in going from 25 to 100 keV, the single and triplet curves go from attractive and repulsive to repulsive and attractive, respectively, suggesting that an oscillatory behavior of the cross section versus energy is possible. These results have important implications for the calculation of atom-atom excitation cross sections in this energy range.