Quasiclassical trajectory study of Mg(3s3p(1)P(1)) plus H-2 reaction on fitted ab initio surfaces

Quasi-classical trajectory calculations for the reaction of Mg(3s3p(1)P(1)) with H-2 are performed on two potential energy surfaces (PES), the excited state (1)A' (or B-1(2) in the C-2v symmetry) in the entrance channel and t he ground state (1)A' (or (1)A(1)) in the exit channel. A many-body expansi on procedure is adopted for the construction of the analytical fit function s from the ab initio results. The title reaction involves a nonadiabatic tr ansition between the two potential surfaces. For simplicity, the transition probability is assumed to be unity when the trajectory goes through the re gion of surface crossing and changes to the lower surface. The calculated t otal collisional deactivation and reaction cross sections decrease with the increase of translational collision energy. The calculated rotational prod uct distributions are characterized by a bimodal feature both fur the MgH v = 0 and 1 states. The trend of bimodality is consistent with the observati on reported in experimental studies. Our inspection of individual trajector ies reveals that the low-rotational and high-rotational populations are cau sed by two distinct reaction pathways. This observation supports our previo us expectation for the microscopic branching via the PES anisotropy. The an gular product distribution indicates that the reaction proceeds predominant ly via a linear collision complex. An increase of the collision energy from 2.026 to 8.104 kcal/mol has resulted in a shift of the distribution toward forward direction. The vibrational product distribution tends to decrease with the quantum numbers. The ratio of MgH(v = 1) to MgH(v = 0) yields a va lue of similar to 0.3, which is nevertheless underestimated as compared wit h the observation of 0.7 +/- 0.2. The reasons for the discrepancy are also discussed.