We measured the temperature dependence of rotational population distri
bution of the nascent product MgH((2) Sigma(+)) in the reaction of Mg(
3s3p(1)P(1)) with H-2. The results indicate that the reaction is domin
ated by an Mg-insertive mechanism, consistent with the isotope effect
reported previously. We also presented the vibrational population dist
ribution, and thereby found that two parallel reaction pathways are re
sponsible for the subject reaction following Mg-H-2 collision in a ben
t configuration. The major one produces MgH in higher rotational level
s and comparable v '' = 0 and v '' = 1 populations, while the other mi
nor one produces MgH in low rotational levels and preferentially v ''
= 0. By means of a two-dimensional potential energy surface(PES) calcu
lation, a deep insight into the reaction pathways has been gained. The
resulting PES's information reveals the possibility of a nonadiabatic
transition between the excited B-1(2) PES and the ground PES. The ben
t intermediate MgH2 near the surface crossing starts trajectories eith
er smoothly following the dissociation coordinate of Mg-H distance or
attractively falling down through a linear HMgH geometry before breaki
ng apart. The former trajectory accounts for the minor reaction pathwa
y to produce MgH, while the latter one responses to the major reaction
pathway. The impact of isotope and temperature effects on MgH can als
o be readily explained with use of the calculated PES's.