Alkali-hydrogen reactions
Journal
International Reviews in Physical Chemistry
Journal Volume
21
Journal Issue
3
Pages
357-383
Date Issued
2002
Author(s)
KING-CHUEN LIN
Vetter, R.
Abstract
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 + H2, Na + H2, K + H2, Rb + H2 and Cs + H2 reactions, in which high-resolution optical techniques have been used for reagent excitation and product characterization. The experimental findings indicate that, in spite of common characteristics, the five reactions exhibit apparently different 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 system plays a major role in the reaction, and not the potential energy above threshold. The way in which the electron transfer occurs along the reaction path is the key factor that influences the collision mechanism: side-on insertion for lithium and sodium and collinear abstraction for potassium, rubidium and caesium. The height of the potential barrier and the size and excitation energy of the alkali atom are also deciding factors. More systematic studies and new experiments are necessary to confirm these trends and to complete our understanding of these model reactions.
SDGs
Type
journal article