Quasi-classical trajectory calculations for the reaction of Mg(3s3p 1 P 1 ) with H 2 are performed on two potential energy surfaces (PES), the excited state 1 A′ (or 1 B 2 in the C 2V symmetry) in the entrance channel and the ground state 1 A′ (or 1 A 1 ) in the exit channel. A many-body expansion procedure is adopted for the construction of the analytical fit functions from the ab initio results. The title reaction involves a nonadiabatic transition between the two potential surfaces. For simplicity, the transition probability is assumed to be unity when the trajectory goes through the region of surface crossing and changes to the lower surface. The calculated total collisional deactivation and reaction cross sections decrease with the increase of translational collision energy. The calculated rotational product distributions are characterized by a bimodal feature both for the MgH V ) 0 and 1 states. The trend of bimodality is consistent with the observation reported in experimental studies. Our inspection of individual trajectories reveals that the low-rotational and high-rotational populations are caused by two distinct reaction pathways. This observation supports our previous expectation for the microscopic branching via the PES anisotropy. The angular product distribution indicates that the reaction proceeds predominantly via a linear collision complex. An increase of the collision energy from 2.026 to 8.104 kcal/ mol has resulted in a shift of the distribution toward forward direction. The vibrational product distribution tends to decrease with the quantum numbers. The ratio of MgH(V ) 1) to MgH(V ) 0) yields a value of ∼0.3, which is nevertheless underestimated as compared with the observation of 0.7 ( 0.2. The reasons for the discrepancy are also discussed.

By using a pump-probe technique, we have observed the nascent rotational population distribution of LiH (v=0) in the Li (2 2PJ) with a H2 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) C2v collision geometry is favored. The Li (2 2PJ)–H2 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 2S or 3 2P), we cannot detect any LiH product. From a theoretical point of view, the 4A′ surface, correlating with the Li 3 2S state, may feasibly couple to a repulsive 3A′ surface, from which the collision complex will rapidly break apart into Li (2 2PJ) and H2. 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 2P) state is expected to behave similarly. The observation also provides indirect evidence that the harpoon mechanism is not applicable to this system.

Ab initio potential energy surfaces and the corresponding analytical energy functions of the ground 1A' and excited 2A' states for the Li(2(2)P) plus H(2) reaction are constructed. Quasiclassical trajectory calculations on the fitted energy functions are performed to characterize the reactions of Li(2(2)P) with H(2)(v = 0, j = 1) and H(2)(v = 1, j = 1) as well as the reaction when the vibrational energy is replaced by collision energy. For simplicity, the transition probability is assumed to be unity when the trajectories go through the crossing seam region and change to the lower surface. The calculated rotational distributions of LiH(v = 0) for both H(2)(v = 0, j = 1) and H(2)(v = 1, j = 1) reactions are single-peaked with the maximum population at j' = 7, consistent with the previous observation. The vibrational excitation of H(2)(v = 1) may enhance the reaction cross section of LiH(v' = 0) by about 200 times, as compared to a result of 93-107 reported in the experimental measurements. In contrast, the enhancement is 3.1, if the same amount of energy is deposited in the translational states. This endothermic reaction can be considered as an analog of late barrier. According to the trajectory analysis, the vibrational excitation enlarges the H-H distance in the entrance channel to facilitate the reaction, but the excess energy may not open up additional reaction configuration.