Cold He+H2 collisions near dissociation

Cross sections for $\mathrm{He}+{\mathrm{H}}_{2}$ collisions are reported for rovibrational states near dissociation at translational energies less than $1000\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$. In contrast to our previously reported cross sections for lower-lying initial rotational states, the excited rotational states near dissociation give rise to shape resonances for energies between 0.001 and $1\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$. The emergence of these resonances with increasing rotational level is opposite to the trend found for $\mathrm{He}+\mathrm{C}\mathrm{O}$ where the shape resonance strength decreases with rotational level for low-lying states. Quasiresonant energy transfer and low energy excitation thresholds are also discussed and it is found that there are 11 rovibrationally excited states that are stable against collision at ultracold temperatures.