The role of conical intersections in the nonadiabatic quenching of OH(A (2)Sigma(+)) by molecular hydrogen

The role of the C-2v(1 B-2(2)-1 (2)A(1)), C-s(1 (2)A'-2 (2)A'), and C-infin ityv(1 (2)Pi -1 (2)Sigma (+)) seams of conical intersection in the dynamics of the nonadiabatic quenching of OH(A (2)Sigma (+)) by molecular hydrogen is investigated. The locus of the C-2v and C-infinityv conical intersection s and the C-s seam bridging them is determined. The accessibility of these seams following optical excitation from the ground 1 B-2(2) state of the OH -H-2 van der Waals complex is examined. Using linear interpolation pathways on the 2 (2)A' potential-energy surface connecting these seams and the Fra nck-Condon region for vertical excitation as a guide, a transition state se parating the 2 (2)A' van der Waals complex from the seam of conical interse ction was located. The barrier of 8.44 kcal mol(-1) is due primarily to the required rotation of the OH molecule relative to H-2. Finally, gradient de scent paths from the conical intersection were determined to identify the p ossible products of the nonadiabatic quenching. For each seam, pathways lea ding to both OH+H-2 and H2O+H products were found. (C) 2000 American Instit ute of Physics. [S0021- 9606(00)00446-3].