Electronic structure and short-range recombination dynamics of S-2 in solid argon

Potential energy curves for 13 lowest electronic states of S-2 and 6 lowest states of ArS are computed at the MRCI level utilizing the CASSCF orbitals . The electronic structure of S-2 is described by the correlation consisten t cc-pVQZ basis set, whereas for ArS the augmented version of this basis is combined with ten electron-core pseudopotential basis set for S and Ar, re spectively. Thermal and shock wave induced recombination dynamics of sulfur atoms trapped in Ar lattice are investigated by classical Molecular Dynami cs simulations. It is observed that atoms separated by nearest neighbor dis tance of the lattice do immediately recombine even at 1 K with no thermal a ctivation. While separated by one lattice constant, the S atoms stay stable up to 80 K and no recombination is observed in the classical trajectories. Consequently, the simulation was able to reproduce the experimental S+S gl ow curve only by lowering the reaction barrier by introducing lattice vacan cies in the four atom plane separating the S-S pair. Local 1.1 eV kinetic e xcitation of a lattice atom next to the trapped S-S pair or artificial shoc k waves initiated by more distant excitation at 2 eV or more greatly enhanc ed the probability for recombination. Nonradiative D-1-P-3 relaxation of su lfur provides a mechanism for such phonon emission and is discussed as a po tential reason for the observed photoinduced recombination in low temperatu re Ar matrix. (C) 2000 American Institute of Physics. [S0021-9606(00)30317- 8].