Using a pump-probe technique, we have systematically studied the state-selected effect on the K-H-2 reaction, showing that the reactivity follows the trend of D < P < S. As long as the system is energetically allowed for reaction, the potential energy is not the key parameter, but the atomic orbital symmetry determines such a state selectivity. The observation of KH(v = 0-3) rotational population in the reaction of K(6s,7s) corresponds to a statistical thermal distribution at 610 +/- 20 K. In contrast, the vibration is highly excited, yielding a Boltzmann vibrational temperature of 2946 +/- 110 and 3150 +/- 200 K. These results provide evidence that the attacking K atom approaches along a collinear geometry, and KH is produced via an ion-pair K+H2- intermediate as a likely pathway. The fraction of product energy partitioning yields 70%, 26%, and 4% for translation, vibration, and rotation. The individual energy disposal into vibration increases with the excitation energy of K. The fact indicates that the electron jumping distance elongates along the order of 5P < 6S < 6P < 7S < 7P, consistent with the prediction by the harpoon mechanism. Most available energy dissipation into translation is caused by a strong instability of the H-2(-) bond. The repulsive energy release from the H-2(-) bond rupture is seriously affected by the attraction between K+ and H-. Therefore, the direct interaction with product repulsion (DIPR) model may not be valid to describe the current system. As ''mixed energy release'' concept is considered instead, a disposal comparison of available energy among the reactions of K-Br-2, K-H-2, and Cs-H-2 may be rationalized. (C) 1997 American Institute of Physics.