THEORETICAL INVESTIGATIONS OF HYDROGEN-ATOM DIFFUSION RATES IN XENON MATRICES

Thermal diffusion rates of hydrogen atoms in a face-centered cubic (fe e) xenon lattice have been computed at 12, 40, and 80 K using a classi cal variational transition-state theory method which employs a Markov walk/damped trajectory procedure to effect convergence. The two-body X e/H interaction is obtained from the results of ab initio calculations at the Moller-Plesset fourth-order perturbation theory level with ah configurations through quadruples included. The calculations employ a double-zeta basis set combined with two different pseudopotentials for the xenon core. Pairwise potentials are generated by fitting the ab i nitio results to a Lennard-Jones (12,6) potential. Standard combining rules are also employed to obtain the Xe/H pairwise interaction as a t est of such procedures. The calculations show that thermal diffusion r ates of hydrogen atoms in fee xenon crystals are very slow with an act ivation energy between 2-3 kcal/mol. The diffusion rates are observed to increase with increasing temperature, as expected. Hydrogen atom tu nneling is the major diffusion process at temperatures around 12 K. At the higher temperatures studied, tunneling is negligible. Comparison of the results with measured diffusion coefficients indicates that nea rly all of the experimentally observed diffusion is occurring along la ttice defects, grain boundaries, vacancies, and other lattice imperfec tions.