A theory of the cross sections of threehyphen;body gashyphen;phase reactions which proceed without activation energy is presented. It is based on the hypothesis that the decomposition of a collision complex is governed by the phase space available to each product under conservation of angular momentum and energy. Calculations on model ionmdash;molecule systems show that the cross section is enhanced if the reaction is exothermic, the final reduced mass is large, and the final longhyphen;range attractive forces are large. The theory gives a consistent explanation for the cross sections of the reactions between He+and H2, and He and H2+which is in satisfactory agreement with experiment.
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