Quantum effects on translation and rotation of molecular chlorine in solid para-hydrogen

Abstract

Structure and quantum effects of a Cl2 molecule embedded in fcc and hcp para-hydrogen (pH2) crystals are are investigated in the zero temperature limit. The interaction is modeled in terms of Cl2-pH2 and pH2-pH2 pair potentials from ab-initio CCSD(T) and MP2 calculations. Translational and rotational motions of the molecules are described within three--dimensional, anharmonic Einstein and Devonshire models, respectively, where the crystals are either treated as frozen or are allowed to relax. The pH2 molecules, as well as the heavier Cl2 molecule, show large translational zero--point energies and undergo large amplitude translational motions. This gives rise to substantial reductions of the cohesive energies and to expansions of the lattices, in agreement with experimental results for pure hydrogen crystals. The rotational dynamics of the Cl2 impurity is restricted to small amplitude librations, again with high librational ZPEs, which are described in terms of two--dimensional non--degenerate anharmonic oscillators. The lattice relaxation causes qualitative changes of the rotational energy surfaces, which finally favor librations around the crystallographic directions pointing towards the nearest--neighbours, both for fcc and hcp lattices. Implications on the reactant orientation in the experimentally observed laser induced chemical reaction, Cl + H2 --> HCl + H, are discussed.