The reaction Li(2 2PJ)+H2(v=1)→LiH(X 1Σ+)+H is studied to understand the effect of vibrational excitation on the reaction pathways. The hydrogen molecules in the v=1, j=0–3 levels are populated by using stimulated Raman pumping (SRP). A pump–probe technique is employed simultaneously to initiate the reaction and monitor the products. The pump–SRP and SRP–probe delay time are short enough to allow for the products to be in a nascent state. The population fraction in the v=1 level can be estimated to be 7.5% by using coherent anti-Stokes Raman spectroscopy. As characterized by Boltzmann rotational temperature of 730 K, the rotational state distributions of LiH(v=0) obtained with H2(v=1) appear to be unimodal, similar to those obtained with H2(v=0), but the product yield is enhanced by a factor of 7–8 times. According to the potential energy surfaces calculations, the insertion mechanism in (near) C2v collision configuration is favored. The Li(2 2PJ)–H2 collision is initially along the 2A′ surface in the entrance channel and then transits to the ground 1A′ surface, from which the products are formed. When H2 stretches to its outer turning point (∼0.9 Å), the 2A′ surface may diabatically couple to the 1A′ surface in the attractive region. An energy barrier of 4300 cm−1 will otherwise obscure the reaction if the H2 bond distance is fixed at 0.75 Å. The energy deposited in the v=1 level simply enlarges the H2 bond distance to help facilitate the reaction and increase the subsequent product yield. The lack of detection of the product LiH(v⩾1) implies that the exoergic energy by 2530 cm−1 should not be distributed statistically among different freedom of motions. The vibrational excitation does not seem to open up an additional pathway for the reaction.
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.
The reactions of alkaline earth metal atoms, Mg(3s3p 1P1) and Ca(4s4p 1P1), with H2(v = 1, j) are studied using a pump-probe technique combined with stimulated Raman pumping and coherent anti-Stokes Raman spectroscopy. For the Ca(4 1P1) case, the energy deposited in the v = 1 level enlarges the H2 bond distance to help facilitate the reaction without opening an additional pathway. For the Mg(3 1P1) case, the vibrational excitation of H2 leads to enhancement of the low rotational component of the rotational distribution and the MgH(v = 0)/MgH(v = 1) ratio. These results can be predicted with quasi-classical trajectory calculations and interpreted with a kinematic collision model.
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