Non-adiabatic coupling in cold collisions of spin-polarized metastable hydrogen atoms

Previous calculations of low temperature cross sections for collisions between metastable hydrogen atoms are improved to include non-adiabatic radial and angular coupling. The electrostatic dipole-quadrupole interaction produces non-adiabatic radial coupling between (2s,2p) and (2p,2p) states while the Coriolis interaction produces non-adiabatic angular coupling. Both of these long-range contributions are handled in a space-fixed atomic gauge that is particularly convenient for a spin-polarized system. The improved theoretical results are compared with an existing experiment.