Lithium impurity recombination in solid para-hydrogen: A path integral quantum transition state theory study

The recombination of two lithium atoms trapped in one-vacancy defect sites of solid para-hydrogen at 4 K and zero external pressure is studied as a quantum activated process. The quantum activation free energy is calculated using path integral quantum transition state theory alone with the method of path integral molecular dynamics simulation. The equilibrium volume of the system is determined by a constant pressure method that scales the sides of the rectangular simulation box. At a fixed equilibrium volume of the system, a constraint dynamics path integral simulation is then employed to determine the quantum path centroid free energy barrier along the reaction coordinate, which is taken to be the relative Li-Li separation. The two Lithium atoms begin to recombine at a distance of approximately twice the lattice spacing, and the height of the barrier relative to the metastable well is 78+/-10 K. The rare of the intrinsic recombination step is estimated to be 1.3 x 10(3) s(-1) at 4 K, It is found that the lithium nuclei exhibit significant tunneling behavior over their classical limit. (C) 1998 American Institute of Physics.