Molecular Beam Kinetics: Reactions of K, Rb, and Cs with Cl2

Crossed beam studies have been made of the reactions of K, Rb, and Cs atoms with Cl₂. These systems are found to exhibit the “stripping” reaction characteristics previously observed for the M + Br₂ and I₂ systems: (1) the reaction cross sections are very large, ⪞150 Ų; (2) most of the alkali halide product recoils into the forward hemisphere with respect to the incident alkali beam, with scattering angle $\text{θ\hspace{0.17em}<\hspace{0.17em}60°}$ (in the center‐of‐mass system). However, there also seems to be substantial intensity (∼5%–15% of the forward peak) throughout the backward hemisphere, $\text{90°\hspace{0.17em}<\hspace{0.17em}θ\hspace{0.17em}<\hspace{0.17em}180°}$ . The sharpness of this forward peaking appears to decrease along the sequence Cl₂, Br₂, I₂ and to be relatively independent of the alkali atom; (3) the angular distribution (in the c.m. system) of alkali atoms scattered without reaction falls off much more rapidly at wide angles than for collisions between unreactive molecules of comparable size. The falloff is most rapid for K + Cl₂ and somewhat less rapid for Rb and Cs + Cl₂, which are roughly comparable with M + Br₂; (4) most of the chemical energy released appears as internal excitation of the products, and other evidence indicates that this is predominantly vibrational excitation of the newly formed bond.