Collisions between Metastable Hydrogen Atoms at Thermal Energies

Abstract

The complex interaction potentials arising in the approach of two metastable hydrogen $2s$ atoms are calculated and the cross sections for ionization, excitation transfer, and elastic scattering are predicted. The measured cross section for associative ionization at $E=4.1\mathrm{meV}$ equals $2\times {10}^{-15\mathrm{}}\mathrm{cm}{}^2$. We calculate a total ionization cross section of $2\times {10}^{-13\mathrm{}}\mathrm{cm}{}^2$, varying as $E^{-2/3\mathrm{}}$ at higher energies. Thus it appears that dissociative ionization is the major ionization channel. We find also that double excitation transfer into two excited $\mathrm{H}\left(2p\right)$ atoms is still more probable with the large cross section of $9\times {10}^{-12\mathrm{}}\mathrm{cm}{}^2$ at $E=4.1\mathrm{meV}$ varying as $E^{-1/2\mathrm{}}$ at higher energies. The detection of the resulting Lyman alpha photons would provide a diagnostic test of our predictions.