Probing the effect of the H-2 rotational state in O(D-1)+H-2 -> OH+H: Theoretical dynamics including nonadiabatic effects and a crossed molecular beam study

Theoretical estimates of reactive cross sections for O(D-1)+H-2(X,upsilon = 0,j)--> OH(X)+H(S-2), with H-2 rotational quantum numbers j=0 and 1, are ob tained for a range of collision energies, E-col. Crossed molecular beam mea surements are also used to infer the ratio, r(1,0), of the j=1 and 0 cross sections at E-col=0.056 eV. The theory indicates that the 1 (1)A' potential surface is the most important one. However, the 2 (1)A' and 1 (1)A' surfac es can also contribute. Adiabatic dynamics on the 1 (1)A' surface, particul arly at E-col above its 0.1 eV barrier to reaction plays a role. The 2 (1)A ' surface, while not correlating with ground electronic state products, can still lead to products via nonadiabatic interactions with the 1 (1)A' surf ace. Many quantum dynamics and quasiclassical classical trajectory calculat ions are carried out. Accurate, ab initio based potential energy surfaces a re employed. Quantum cross sections are based on helicity decoupled wave pa cket calculations for several values of total angular momentum. Nonadiabati c wave packet and trajectory surface hopping calculations, where appropriat e, are carried out. An interesting, subtle picture emerges regarding the en ergy dependence of r(1,0). The theoretical results indicate, somewhat surpr isingly, that, for E-col<0.1 eV,r(1,0) can be less than unity owing to the anisotropy of the ground state potential. Electronically excited states and nonadiabatic effects contribute to the overall cross sections for E-col>0. 1 eV, but the full r(1,0) is only weakly sensitive to excited states. Our e xperimentally inferred r(1,0) at E-col=0.056 eV, 0.95 +/-0.02, is in quanti tative agreement with our best calculation, which suggests that the effect of potential anisotropy is correctly described by theory. The relation betw een these results and previous experimental findings is discussed. (C) 2000 American Institute of Physics. [S0021-9606(00)02041-9].