Vibration-vibration and vibration-translation energy transfer in H ₂-H₂ collisions: A critical test of experiment with full-dimensional quantum dynamics

Quantum scattering calculations of vibration-vibration (VV) and vibration-translation (VT) energy transfer for non-reactive H₂-H ₂ collisions on a full-dimensional potential energy surface are reported for energies ranging from the ultracold to the thermal regime. The efficiency of VV and VT transfer is known to strongly correlate with the energy gap between the initial and final states. In H₂(v=1, j=0) H ₂(v=0, j=1) collisions, the inelastic cross section at low energies is dominated by a VV process leading to H₂(v=0, j=0) H ₂(v=1, j=1) products. At energies above the opening of the v=1, j=2 rotational channel, pure rotational excitation of the para-H₂ molecule leading to the formation of H₂(v=1, j=2) H₂(v=0, j=1) dominates the inelastic cross section. For vibrationally excited H ₂ in the v 2 vibrational level colliding with H₂(v=0), the efficiency of both VV and VT process is examined. It is found that the VV process leading to the formation of 2H₂(v=1) molecules dominates over the VT process leading to H₂(v=1) H₂(v=0) products, consistent with available experimental data, but in contrast to earlier semiclassical results. Overall, VV processes are found to be more efficient than VT processes, for both distinguishable and indistinguishable H ₂-H₂ collisions confirming room temperature measurements for v=1 and v 2.