By using a pump-probe technique, we have observed the nascent rotational population distribution of LiH (v=0) in the Li (2 P-2(J)) with a H-2 reaction, which is endothermic by 1680 cm(-1). The LiH (v=0) distribution yields a single rotational temperature at similar to 770 K, but the population in the v=1 level is not detectable. According to the potential energy surface (PES) calculations, the insertion mechanism in (near) C-2v collision geometry is favored. The Li (2 P-2(J))-H-2 collision is initially along the 2A' surface in the entrance channel and then diabatically couples to the ground 1A' surface, from which the products are formed. From the temperature dependence measurement, the activation energy is evaluated to be 1280 +/- 46 cm(-1), indicating that the energy required for the occurrence of the reaction is approximately the endothermicity. As Li is excited to higher states (3 S-2 or 3 P-2), we cannot detect any LiH product. From a theoretical point of view, the 4A' surface, correlating with the Li 3 S-2 state, may feasibly couple to a repulsive 3A' surface, from which the collision complex will rapidly break apart into Li (2 P-2(J)) and H-2. The probability for further surface hopping to the 2A(') or 1A(') surfaces is negligible, since the 3A(') and 2A(') surfaces are too far separated to allow for an efficient coupling. The Li (3 P-2) state is expected to behave similarly. The observation also provides indirect evidence that the harpoon mechanism is not applicable to this system.