AB-INITIO CALCULATION FOR POTENTIAL-ENERGY SURFACES RELEVANT TO THE MICROSCOPIC REACTION PATHWAYS FOR MG(3S3P(1)P(1))-2-]MGH((2)SIGMA(+))+H(H)

Two ab initio methods have been employed to calculate the dynamical po tential energy surfaces (PES's) for the excited (B-1(2) Or (1)A') and the ground ((1)A(1) or (1)A') states in the Mg(3s3p(1)P(1),)-H-2 react ion. The obtained PES's information reveals that the production of MgH in the (2) Sigma(+) state, as Mg(P-1(1)) approaches H-2 in a bent con figuration, involves a nonadiabatic transition. The MgH2 intermediate around the surface crossing then elicits two distinct reaction pathway s. In the first one, the bent intermediate, affected by a strong aniso tropy of the interaction potential, decomposes via a Linear HMgH geome try. The resulting MgH is anticipated to populate in the quantum state s of rotational and vibrational excitation. In contrast, the second pa thway produces MgH in the low rotational and vibrational states, as a result of the intermediate decomposition along the stretching coordina te of the Mg-H elongation. These two tracks may account for the previo us experimental findings for the MgH distribution, which the impulsive model has failed to comprehend. By far, different interpretations hav e been proposed especially for the low-N MgH product. The supply of a detailed PES's information in this work helps to clarify the ambiguity . It is also conducive to an interpretation of the isotope and tempera ture effects on the product rotational distribution. (C) 1998 American Institute of Physics. [S0021-9606(98)00104-4].