Ultracold homonuclear and heteronuclear collisions in metastable helium

Scattering and ionizing cross sections and rates are calculated for ultracold collisions between metastable helium atoms using a fully quantum-mechanical close-coupled formalism. Homonuclear collisions of the bosonic He∗4+He∗4 and fermionic He∗3+He∗3 systems, and heteronuclear collisions of the mixed He∗3+He∗4 system, are investigated over the temperature range 1 μK to 1 K. Carefully constructed Born-Oppenheimer molecular potentials are used to describe the electrostatic interaction between the colliding atoms, and complex optical potentials to represent loss through ionization from the 1,3ς states. Magnetic spin-dipole-mediated transitions from the 5ς state are included and results reported for spin-polarized and unpolarized systems. Comparisons are made with experimental results, previous semiclassical models, and a perturbed single-channel model.