MOLECULAR BEAM KINETICS - TRANSITION BETWEEN REBOUND AND STRIPPING MECHANISMS IN REACTIONS OF ALKALI ATOMS WITH POLYHALIDE MOLECULES

Angular distribution measurements have been made for reactive and elastic scattering of K, Rb, or Cs atoms from two series of polyhalide molecules (listed in order of decreasing reaction yield) : PBr3>PCl3≫ BBr3; and CBr4SnCl4>CCl4> CHCl3>SiCl4. For PBr3, PCl3, CBr4, and SnCl4 the reaction cross sections are very large (150 Å2) and stripping behavior is predominant: most of the alkali halide recoils into the forward hemisphere with respect to the incident alkali-atom beam. For CCl4 the laboratory alkali halide distributions are bimodal, corresponding in the center-of-mass system to conical, sideways-peaked distributions of the recoil velocity vectors. The half-angle of the cone is roughly 50° for Cs, 70° for Rb, and 90° for K. For CHCl3 the alkali halide distribution favors the backward hemisphere. The angular distribution of alkali atoms scattered without reaction likewise shows a strong correlation with the reaction yield. As compared with the much less reactive reference systems BBr3 and SiCl4, the elastic scattering at wide angles falls off more and more rapidly as the reaction yield increases; beyond 90° the elastic intensity for SnCl 4 is only 2% of that for SiCl4. Regardless of the size of the reaction cross section, however, the total scattering (alkali atoms plus alkali halide) is very similar in both magnitude and angular distribution to that for the reference molecules. The variations in reactivity agree well with the early Polanyi flame results and are also qualitatively correlated with the electron-capture coefficients of the reactant molecules.