Collisions between Metastable Hydrogen Atoms at Thermal Energies

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 E0ex0ex=0ex0ex4.10ex0exmeV equals 2×10^-150ex0excm². We calculate a total ionization cross section of 2×10^-130ex0excm², varying as E^-2/3 at higher energies. Thus it appears that dissociative ionization is the major ionization channel. We find also that double excitation transfer into two excited H(2p) atoms is still more probable with the large cross section of 9×10^-120ex0excm² at E0ex0ex=0ex0ex4.10ex0exmeV varying as E^-1/2 at higher energies. The detection of the resulting Lyman alpha photons would provide a diagnostic test of our predictions.