CLASSICAL VARIATIONAL TRANSITION-STATE THEORY STUDY OF HYDROGEN-ATOM DIFFUSION DYNAMICS IN IMPERFECT XENON MATRICES

Thermal diffusion rates of hydrogen atoms in imperfect face-centered-c ubic (fee) xenon lattices containing up to 4.12% vacant sites have bee n computed using classical Monte Carlo variational transition state th eory with a pairwise Xe/H interaction potential obtained from the resu lts of ab initio calculations at the MP4(SDTQ) level of theory. Conver gence of the required integrals is achieved by combining importance sa mpling and a damped trajectory procedure with the standard Markov walk . The variational flux through spherical dividing surfaces is minimize d as a function of radius of the dividing surface. The results show th at the presence of 1.4% vacant lattice sites lowers the diffusion barr ier by about 0.006 eV relative to the perfect fee crystal system. The computed values of the hydrogen atom diffusion coefficients at 40 K in dicate that, over the range of vacancies considered, the diffusion coe fficients increase exponentially with the percentage of the lattice va cancies. The calculations also show that the lattice vacancies are mob ile. The studies reveal that the propensity for vacant site mobility i ncreases as the total number of lattice vacancies increases. Since thi s effect decreases the potential barrier to diffusion, the diffusion c oefficients obtained from the variational transition state theory calc ulation are lower limits for a system with the present interaction pot ential. The calculated diffusion coefficients indicate that experiment al matrices vapor-deposited at 10 and 28 K contain about 1.8 and 1.2% vacant sites, respectively. Since the calculated diffusion rates are l ower limits, these percentages are upper limits for the potential surf ace used in the present investigation.