Rotational Feshbach Resonances in Ultracold Molecular Collisions

In collisions at ultralow temperatures, molecules will possess Feshbach resonances, foreign to ultracold atoms, whose virtual excited states consist of rotations of the molecules. We estimate the mean spacing and mean widths of these resonant states, exploiting the fact the molecular collisions at low energy display chaotic motion. As examples, we consider the experimentally relevant molecules ${\mathrm{O}}_{\mathrm{2}}$, OH, and PbO. Especially for polar species, the density of $s$-wave resonant states is quite high, implying potentially disastrous consequences for trapped molecules.