Classical Variational Transition State Theory Study of Hydrogen Atom Diffusion Dynamics in Imperfect Xenon Matrices

Abstract

Thermal diffusion rates of hydrogen atoms in imperfect face-centered-cubic (fcc) xenon lattices containing up to 4.12% vacant sites have been computed using classical Monte Carlo variational transition state theory with a pairwise Xe/H interaction potential obtained from the results of ab initio calculations at the MP4(SDTQ) level of theory. Convergence of the required integrals is achieved by combining importance sampling and a damped trajectory procedure with the standard Markov walk. The variational flux through spherical dividing surfaces is minimized as a function of radius of the dividing surface. The results show that the presence of 1.4% vacant lattice sites lowers the diffusion barrier by about 0.006 eV relative to the perfect fcc crystal system. The computed values of the hydrogen atom diffusion coefficients at 40 K indicate that, over the range of vacancies considered, the diffusion coefficients increase exponentially with the percentage of the lattice vacancies. The calculations also show that the lattice vacancies are mobile. The studies reveal that the propensity for vacant site mobility increases as the total number of lattice vacancies increases. Since this effect decreases the potential barrier to diffusion, the diffusion coefficients obtained from the variational transition state theory calculation are lower limits for a system with the present interaction potential. The calculated diffusion coefficients indicate that experimental matrices vapor-deposited at 10 and 28 K contain about 1.8 and 1.2% vacant sites, respectively. Since the calculated diffusion rates are lower limits, these percentages are upper limits for the potential surface used in the present investigation.