Exchange reactions of alkali atoms with alkali halides: a collision complex mechanism

Abstract

Molecular beam studies of the reactions of Cs and K with RbCl and the reverse reactions are described. Use of a mass spectrometer with a surface ionization filament provides differential detection of both of the product species and both of the reactants. The results show that these reactions form collision complexes with lifetime long compared to a rotational period. In the centre-of-mass reference system, the angular distribution of products has symmetry about θ= 90° and peaks very strongly near θ= 0 and 180°. Furthermore, at wide angles the chemically elastic scattering shows a “sticky collision bump” arising from break-up of the complex to reform the reactants rather than the products. Both the products and the reformed reactants emerge from the complex with a translational temperature appreciably higher than that for the approach of the parent beams (e.g., Tout≃1600°K; Tin≃800°K for Cs + RbCl).The shape of the angular distributions is interpreted in terms of a statistical model adapted from the compound nucleus treatment of nuclear fission. The strong peaking of I(θ) indicates that the complex forms and dissociates with orbital angular momenta much larger than the rotational momenta of the reactant or product salt molecules. The complex thus must be formed with high probability in collisions with impact parameters as large as 8-9 A. This is consistent with calculations which assume that the cross section is governed by the long-range dipole-induced dipole interaction and that most collisions which surmount the centrifugal barrier lead to complex formation. A rough lower limit of 5 × 10–12 sec is estimated for the mean lifetime of the complex.