Quantum and classical quasiresonant vibration-rotation energy transfer is investigated for ultracold $\mathrm{He}-{\mathrm{H}}_2$ collisions. Classical trajectory computations show that extremely strong correlations between $\Delta j$ and $\Delta v$ persist at low energies, though the changes themselves are less than one quantum. Quantum computations show that quasiresonant transitions occur in the limit of zero collision energy but that threshold effects become important and that some quasiresonant channels close. The qualitative similarity between classical and quantum results suggests that they share a common mechanism.

• Received 29 October 1998

DOI:https://doi.org/10.1103/PhysRevLett.82.2657