  A potential energy surface for the lowest quartet electronic state of lithium
trimer is developed and used to study spin-polarized Li + Li_2 collisions at
ultralow kinetic energies. The potential energy surface allows barrierless atom
exchange reactions. Elastic and inelastic cross sections are calculated for
collisions involving a variety of rovibrational states of Li_2. Inelastic
collisions are responsible for trap loss in molecule production experiments.
Isotope effects and the sensitivity of the results to details of the potential
energy surface are investigated. It is found that for vibrationally excited
states the cross sections are only quite weakly dependent on details of the
potential energy surface.
