Reaction pathway, energy barrier, and rotational state distribution for Li(2 P-2(J))+H-2 -> LiH (X (1)Sigma(+))+H

By using a pump-probe technique, we have observed the nascent rotational po pulation 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 th e v=1 level is not detectable. According to the potential energy surface (P ES) 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' sur face in the entrance channel and then diabatically couples to the ground 1A ' surface, from which the products are formed. From the temperature depende nce measurement, the activation energy is evaluated to be 1280 +/- 46 cm(-1 ), indicating that the energy required for the occurrence of the reaction i s 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 co uple to a repulsive 3A' surface, from which the collision complex will rapi dly break apart into Li (2 P-2(J)) and H-2. The probability for further sur face 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 als o provides indirect evidence that the harpoon mechanism is not applicable t o this system. (C) 2001 American Institute of Physics.