Nonadiabatic Atom-Atom Collisions: Two-State Impact-Parameter Calculation of theH(1s)+H(1s)=H(1s)+H(2s)Cross Section Including Electron Exchange

The cross section for the $1s-2s$ excitation of hydrogen atoms by incident hydrogen atoms is calculated for laboratory energies 1-56.25 keV. The calculation is made in the coupled-state impact-parameter approximation, including electron exchange. All exchange potentials are calculated as a function of internuclear distance and velocity. The cross section is much larger at low energies than that predicted by an unsymmetrized state expansion, and at higher energies shows a very smooth undulatory behavior as a function of velocity. The calculation establishes the importance of electron exchange for incident energies at least as high as 100 keV.