Collisions and reactions of ultracold molecules

It is argued that collision dynamics of atoms and molecules at ultracold temperatures (below 1 mK) are not readily predictable from knowledge of collision dynamics above 100 K. In the case of elastic collisions, it is well known that the collision cross section is constant as T --> 0 K but mass and symmetry effects are dramatic. The cases of inelastic and reactive collisions are less studied, but a T-1/2 dependence of the cross section as T --> 0 K is expected. It seems that extrapolations of high-temperature inelastic and reactive behavior normally greatly underestimate ultracold-temperature rates. The prospects for experimental observation of ultracold collision dynamics are rapidly improving.