The nascent CaH product in the reaction Ca(4s4p P-1(1))+H-2-->CaH(X(2)Sigma(+))+H is obtained using a pump-probe technique. The CaH(v=0,1) distributions, with a population ratio of CaH(v=0)/CaH(v=1)=2.7+/-0.2, may be characterized by low Boltzmann rotational temperature. According to Arrhenius theory, the temperature dependence measurement yields a potential barrier of 3820+/-480 cm(-1) for the current reaction. As a result of the potential energy surfaces (PES) calculations, the reaction pathway favors a Ca insertion into the H-2 bond along a (near) C-2v geometric approach. As the H-2 bond is elongated, the configurational mixing between the orbital components of the 4p and nearby low-lying 3d state with the same symmetry makes significant the nonadiabatic transition between the 5A' and 2A' surface in the repulsive limbs. Therefore, the collision species are anticipated to track along the 5A' surface, then undergo nonadiabatic transition to the inner limb of the 2A' surface, and finally cross to the reactive 1A' surface. The observed energy barrier probably accounts for the energy requirement to surmount the repulsive hill in the entrance. The findings of the nascent CaH product distributions may be reasonably interpreted from the nature of the intermediate structure and lifetime after the 2A'-1A' surface transition. The distinct product distributions between the Ca(4(1)P(1)) and Mg(3(1)P(1)) reactions with H-2 may also be realized with the aid of the PES calculations. (C) 2004 American Institute of Physics.