Rotational relaxation of HF by collision with ortho- and para-H2 molecules.

The first quantum mechanical investigation of the rotational deactivation of HF induced by collisions with ortho- and para-H(2) molecules is reported. Ab initio potential energy calculations are carried out at the coupled cluster level with single and double excitations, using a quadruple-zeta basis set. The global rigid rotor four-dimensional potential energy surface is obtained by fitting ab initio points with a least squares procedure for the angular terms and interpolating the radial coefficients with cubic splines. It is shown that the equilibrium structure of the H(2)-HF complex is T-shaped and the well depth is found to be 359 cm(-1). Close coupling scattering calculations are performed at collision energy ranging from 10(-2) to 1600 cm(-1). A comparison of the rotational quenching of HF with para-H(2) and (4)He is used to validate our potential energy surface. The rotational quenching cross sections of HF by ortho- and para-H(2) are also compared and found to be very different. An explanation of these differences based on a resonance mechanism is proposed.