Alkali±hydrogen reactions.pdf

Int . Reviews in Physical Chemistry, 2002 Vol . 21, No . 3, 357± 383 Alkali±hydrogen reactions KING-CHUEN LIN Department of Chemistry, National Taiwan University , and Institute of Atomic and Molecular Sciences, Academi a Sinica, Taipei 106, Taiwan RAYMOND VETTER Laboratoire Aime Cotton, Centre National de la Recherche Scienti®que, Baà timent 505, Campus d’Orsay, 91405 Orsay Cedex, France Highly endoergic alkali±hydrogen reactions are usually initiated by electronic excitation of the alkali atom. This implies that non-adiabatic couplings take place along the reaction path, the collision mechanism being dominated by the electron transfer jump that occurs between the alkali atom and the hydrogen molecule. Since electronic excitation can be selectively achieved over many atomic states, these reactions constitute good candidates to generalize the study of non-adiabatic processes in reaction dynamics. Series of heat-pipe oven and crossed-beam experiments have been carried out over the Li ‡ H , Na ‡ H , K ‡ H , Rb ‡ H 2 2 2 2 and Cs ‡ H2 reactions, in which high-resolution optical techniques have been used for reagent excitation and product characterization. The experimental ®ndings indicate that, in spite of common characteristics, the ®ve reactions exhibit apparently di erent behaviours that are not explained by usual collision models. However, a critical analysis based on the few calculations of ab initio potential energy surfaces and collision dynamics shows that the electronic symmetry of the reagent±product