Molecular beam kinetics: Long-lived collision complexes in reactions of K, Rb, and Cs with SnCl4 and SF6

Velocity distributions of reactive scattering from crossed thermal beams of K, Rb, or Cs and SnCl4 or SF6 have been measured over a wide angular range. A kinematic transformation of the data to the center-of-mass coordinate system provides contour maps of the differential reaction cross sections. The cross sections show approximately symmetrical forward-backward peaking with respect to the initial relative velocity vector and very low product translational energy. These features indicate the reactions proceed via formation and decay of a collision complex that persists for many vibrational periods and at least several rotational periods. The observed product angular and velocity distributions are shown to be consistent with a simple transition-state theory developed from the RRKM treatment of unimolecular decomposition. For the SnCl4 reactions, the cross sections give evidence of two distinct decay modes, one consistent with formation of alkali chloride and the other a much heavier alkali compound, probably the alkali chlorostannite, MSnCl3. The latter mode accounts for ∼13 of the reactive scattering for the K case, ∼14 for Rb, and ∼15 for Cs.