THEORETICAL INVESTIGATIONS OF O3 VIBRATIONAL-RELAXATION AND OXYGEN-ATOM DIFFUSION RATES IN AR AND XE MATRICES

The molecular dynamics of vibrationally-excited ozone isolated in Ar a nd Xe matrices at 12 K are investigated using classical trajectory met hods. Oxygen atom diffusion in these matrices are computed using a cla ssical variational transition-state theory which employs a new Markov walk/damped trajectory procedure to effect convergence. The matrix mod el consists of a face-centered-cubic (fcc) crystal with 125 unit cells (666 atoms) in a cubic (5 X 5 X 5) arrangement. Ozone, or the oxygen atom, is held interstitially within the innermost unit cell of the cry stal. The system potential is written as the sum of a lattice potentia l, a lattice-ozone or oxygen atom interaction, and a gas-phase potenti al for ozone. The first two potentials are assumed to have pairwise fo rm, while the ozone molecular potential is the one developed by Murrel l and Farantos [Mol. Phys. 1977, 34, 1185]. The oxygen atom/Xe pair po tentials are computed at the Hartree-Fock and Moller-Plesset second-or der perturbation level of theory for the singlet and triplet states us ing two different pseudopotentials for the xenon core. Vibrational rel axation is observed to be mode specific with the bending mode dominati ng the rate of energy transfer to the lattice. The rates over the firs t 0. 1-0.2 ps are characterized by near-linear, first-order decay. The energy-transfer rates are found to be significantly faster in Ar than in Xe matrices. Thermal diffusion rates of oxygen atoms in fcc Xe cry stals are computed for a range of possible O/Xe Lennard-Jones (12,6) i nteraction potentials suggested by the pseudopotential calculations. I n all cases, diffusion is found to be extremely slow with an activatio n energy greater than 3.3 kcal/mol. Quantum tunneling is shown to make a negligibly small contribution to these rates. The corresponding dif fusion rates in Ar are even slower. Comparison of the results with mea sured diffusion coefficients indicates that almost all of the experime ntally observed diffusion is occurring along lattice defects, grain bo undaries, vacancies, and other lattice imperfections.