We present a rigorous study of vibrational relaxation in $\text{para}\text{−}{\mathrm{H}}_2+\text{para}\text{−}{\mathrm{H}}_2$ collisions at cold and ultracold temperatures and identify an efficient mechanism of rovibrational energy transfer. If the colliding molecules are in different rotational and vibrational levels, the internal energy may be transferred between the molecules through an extremely state-selective process involving simultaneous conservation of internal energy and total rotational angular momentum. The same transition in collisions of distinguishable molecules corresponds to the rotational energy transfer from one vibrational state of the colliding molecules to another.

• Received 14 September 2007

DOI:https://doi.org/10.1103/PhysRevA.77.030704