. at om - p h ] 2 3 M ay 2 00 1 Ultracold collisions of oxygen molecules

Collision cross sections and rate constants between two groundstate oxygen molecules are investigated theoretically at translational energies below ∼ 1K and in zero magnetic field. We present calculations for elastic and spinchanging inelastic collision rates for different isotopic combinations of oxygen atoms as a prelude to understanding their collisional stability in ultracold magnetic traps. A numerical analysis has been made in the framework of a rigidrotor model that accounts fully for the singlet, triplet, and quintet potential energy surfaces in this system. The results offer insights into the effectiveness of evaporative cooling and the properties of molecular BoseEinstein condensates, as well as estimates of collisional lifetimes in magnetic traps. Specifically, O2 looks like a good candidate for ultracold studies, while O2 is unlikely to survive evaporative cooling. Since O2 is representative of a wide class of molecules that are paramagnetic in their ground state we conclude that many molecules can be successfully magnetically trapped at ultralow temperatures. Typeset using REVTEX 1