Reaction pathway, energy barrier, and rotational state distribution for Li (2 [sup 2]P[sub J])+H[sub 2]→LiH (X [sup 1]Σ[sup +])+H.

By using a pump-probe technique, we have observed the nascent rotational population distribution of LiH (v=0) in the Li (2 [sup 2]P[sub J]) with a H[sub 2] reaction, which is endothermic by 1680 cm-1. The LiH (v=0) distribution yields a single rotational temperature at ∼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[sub 2v] collision geometry is favored. The Li (2 [sup 2]P[sub J])-H[sub 2] collision is initially along the 2A[sup ′] surface in the entrance channel and then diabatically couples to the ground 1A[sup ′] surface, from which the products are formed. From the temperature dependence measurement, the activation energy is evaluated to be 1280±46 cm[sup -1], indicating that the energy required for the occurrence of the reaction is approximately the endothermicity. As Li is excited to higher states (3 [sup 2]S or 3 [sup 2]P), we cannot detect any LiH product. From a theoretical point of view, the 4A[sup ′] surface, correlating with the Li 3 [sup 2]S state, may feasibly couple to a repulsive 3A[sup ′] surface, from which the collision complex will rapidly break apart into Li (2 [sup 2]P[sub J]) and H[sub 2]. The probability for further surface hopping to the 2A[sup ′] or 1A[sup ′] surfaces is negligible, since the 3A[sup ′] and 2A[sup ′] surfaces are too far separated to allow for an efficient coupling. The Li (3 [sup 2]P) state is expected to behave similarly. The observation also provides indirect evidence that the harpoon mechanism is not applicable to this system. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]