Near-resonant rotational energy transfer in HCl-H2 inelastic collisions.

We present a new four-dimensional (4D) potential energy surface for the HCl-H2 van der Waals system. Both molecules were treated as rigid rotors. Potential energy surface was obtained from electronic structure calculations using a coupled cluster with single, double, and perturbative triple excitations method. The four atoms were described using the augmented correlation-consistent quadruple zeta basis set and bond functions were placed at mid-distance between the HCl and H2 centers of mass for a better description of the van der Waals interaction. The global minimum is characterized by the well depth of 213.38 cm(-1) corresponding to the T-shape structure with H2 molecule on the H side of the HCl molecule. The dissociation energies D0 are 34.7 cm(-1) and 42.3 cm(-1) for the complex with para- and ortho-H2, respectively. These theoretical results obtained using our new PES are in good agreement with experimental values [D. T. Anderson, M. Schuder, and D. J. Nesbitt, Chem. Phys. 239, 253 (1998)]. Close coupling calculations of the inelastic integral rotational cross sections of HCl in collisions with para-H2 and ortho-H2 were performed at low and intermediate collisional energies. Significant differences exist between para- and ortho-H2 results. The strongest collision-induced rotational HCl transitions are the transitions with Δj = 1 for collisions with both para-H2 and ortho-H2. Rotational relaxation of HCl in collision with para-H2 in the rotationally excited states j = 2 is dominated by near-resonant energy transfer.