Quantum mechanical investigation of rovibrational relaxation of H2 and D2 by collisions with Ar atoms

We report quantum mechanical calculations of cross sections and rate coefficients for rovibrational relaxation of H2 and D2 by collisions with Ar atoms over a wide range of temperatures including the ultracold limit. Limiting values of the rate coefficients for vibrational and rotational quenching at zero temperature were computed and sensitivity of the results to the choice of the interaction potential is investigated. We also demonstrate dramatic change in the behavior of the rate coefficients at low temperatures when the van der Waals potential supports a quasibound level very close to the dissociation threshold.