Ultracold collisions between two light indistinguishable diatomic molecules: Elastic and rotational energy transfer in HD+HD

A close coupling quantum-mechanical calculation is performed for rotational energy transfer in a HD+HD collision at very low energy, down to the ultracold temperatures: T∼10 -8 K. A global six-dimensional H 2-H 2 potential-energy surface is adopted from a previous work. State-resolved integral cross sections σ ij→i ′j ′( kin) of different quantum-mechanical rotational transitions ij→i ′j ′ in the HD molecules and corresponding state-resolved thermal rate coefficients k ij→i ′j ′(T) have been computed. Additionally, for comparison, H 2+H 2 calculations for a few selected rotational transitions have also been performed. The hydrogen and deuterated hydrogen molecules are treated as rigid rotors in this work. A pronounced isotope effect is identified in the cross sections of these collisions at low and ultracold temperatures. © 2012 American Physical Society.